- remove make target for wmdep (flex-based scanner), which eliminates
  a bootstrap dependency on flex.

  As of OpenFOAM-v1806, wmdep has been superseded by wmdepend
  (ragel-based scanner).

- replace dirToString binary with shell/awk equivalent for simpler
  maintenance. The utility is very rarely used (auto scanning to
  create Make/files) so there is no performance difference.

Feature vof update solvers

See merge request OpenFOAM-plus!266

Integration of VOF MULES new interfaces. Update of VOF solvers and all instances
of MULES in the code.
Integration of reactingTwoPhaseEuler and reactingMultiphaseEuler solvers and sub-models
Updating reactingEuler tutorials accordingly (most of them tested)

New eRefConst thermo used in tutorials. Some modifications at thermo specie level
affecting mostly eThermo. hThermo mostly unaffected

New chtMultiRegionTwoPhaseEulerFoam solver for quenching and tutorial.

Phases sub-models for reactingTwoPhaseEuler and reactingMultiphaseEuler were moved
to src/phaseSystemModels/reactingEulerFoam in order to be used by BC for
chtMultiRegionTwoPhaseEulerFoam.

Update of interCondensatingEvaporatingFoam solver.

- if surface is only on a single processor, no point merging occurs
  and the pointsMap is empty.

- overload the ref() method to allow modification of the referenced
  (non-const) field directly. Same as constCast(), but less typing
  and less prone to error.

- construct ConstPrecisionAdaptor from tmp for improved efficiency.

- the updated surface writers must be explicitly tagged as being
  Time-aware (useTimeDir) to have them splice Time (in this case freq)
  into the output path.

  The older writers worked in the opposite way. They extracted a time
  value from the naming of the output directory (which was fragile).

Feature single precision

See merge request OpenFOAM-plus!234

- with the changes added in #1256 and corresponding updates to
  ThirdParty we can stop exporting these variables:

  WM_CC WM_CFLAGS WM_CXX WM_CXXFLAGS WM_LDFLAGS

- options '-n' and '-np' are synonymous, but some mpiexec versions may
  only recognize '-n' and not '-np'

- need additional tolerance when comparing time values to those
  stored in the fieldDict (both less and equal operators)

ENH: improved handling of regIOobject deletion (#1276)

See merge request OpenFOAM-plus!263

ENH: FO: Lamb vector and its divergence

See merge request OpenFOAM-plus!245

  - If applied:
      This commit allows the user to compute:
        - the Lamb vector (https://en.wikipedia.org/wiki/Lamb_vector),
        - on-the-fly or via postProcess utility
        - for a given volVectorField (one per functionObject entry)

  - Why:
      The motivation is the literature-reported quantitative connection
      between the Lamb vector (divergence) and the spatially localised
      instantaneous fluid motions, e.g. high- and low-momentum fluid
      parcels, which possess considerable level of capacity to affect
      the rate of change of momentum, and to generate forces such as drag.

  - Verification:
      - Smooth-wall plane channel flow case (Moser et al. 1999) by
          # Curtis et al. (2008) On the Lamb vector divergence
            in Navier–Stokes flows, doi:10.1017/S0022112008002760

  - What's next:
      - The verification-show case
      - Extended code guide entry titled "Lamb vector"

- remove registration and ownership before deleting a regIOobject
  from within objectRegistry to avoid possible recursion.

- regIOobject destructor now removes any registered object from
  registry regardless if ownedByRegistry or not. It also removes
  always removes the ownership flag to avoid possible recursion.

- the regIOobject::checkOut() now unconditionally clears file watches.
  These will only be there if the object is registered (a no-op for an
  unregistered object), but this additional safety is needed to manage
  case where the registration has been modified elsewhere (eg, by the
  objectRegistry).

Feature complex cleanup

See merge request OpenFOAM-plus!264

ENH: define addition/subtraction operations for scalar and complex

- required since construct complex from scalar is explicit
- additional tests in Test-complex

- can additionally relinquish the registered state as well as
  ownedByRegistry state

- when a single (non-regex) source field is specified, it is now
  possible to specify the result name without any '@@' substitution
  tokens

- forces c++DBUG='-DFULLDEBUG -g -O0' for the compilation, to allow
  localized debugging during development without file editing and
  while retaining the WM_COMPILE_OPTION (eg, Opt)

  Note that switching between 'wmake' and 'wmake -debug' will not
  cause existing targets to be rebuilt. As before, these are driven by
  the dependencies. An intermediate wclean may thus be required.

- in case an individual geometry element has render issues in parallel

- additional debug information