Provides run-time selection of buoyancy sources for compressible solvers

Replaces the built-in buoyancy sources in XiFoam, reactingFoam and
rhoReactingFoam.

e.g. in constant/fvOptions specify

momentumSource
{
    type            buoyancyForce;

    buoyancyForceCoeffs
    {
        fieldNames      (U);
    }
}

and optionally specify the buoyancy energy source in the enthalpy
equation:

energySource
{
    type            buoyancyEnergy;

    buoyancyEnergyCoeffs
    {
        fieldNames      (h);
    }
}

or internal energy equation

energySource
{
    type            buoyancyEnergy;

    buoyancyEnergyCoeffs
    {
        fieldNames      (e);
    }
}

Added calls to setFluxRequired for p, p_rgh etc. in all solvers which
avoids the need to add fluxRequired entries in fvSchemes dictionaries.

Now the specification of the LTS time scheme is simply:

ddtSchemes
{
    default         localEuler;
}

Select LTS via the ddtScheme:

    ddtSchemes
    {
        default         localEuler rDeltaT;
    }

Resolves bug-report http://www.openfoam.org/mantisbt/view.php?id=1706

For multi-region cases the default location of blockMeshDict is now system/<region name>

If the blockMeshDict is not found in system then the constant directory
is also checked providing backward-compatibility

The old separate incompressible and compressible libraries have been removed.

Most of the commonly used RANS and LES models have been upgraded to the
new framework but there are a few missing which will be added over the
next few days, in particular the realizable k-epsilon model.  Some of
the less common incompressible RANS models have been introduced into the
new library instantiated for incompressible flow only.  If they prove to
be generally useful they can be templated for compressible and
multiphase application.

The Spalart-Allmaras DDES and IDDES models have been thoroughly
debugged, removing serious errors concerning the use of S rather than
Omega.

The compressible instances of the models have been augmented by a simple
backward-compatible eddyDiffusivity model for thermal transport based on
alphat and alphaEff.  This will be replaced with a separate run-time
selectable thermal transport model framework in a few weeks.

For simplicity and ease of maintenance and further development the
turbulent transport and wall modeling is based on nut/nuEff rather than
mut/muEff for compressible models so that all forms of turbulence models
can use the same wall-functions and other BCs.

All turbulence model selection made in the constant/turbulenceProperties
dictionary with RAS and LES as sub-dictionaries rather than in separate
files which added huge complexity for multiphase.

All tutorials have been updated so study the changes and update your own
cases by comparison with similar cases provided.

Sorry for the inconvenience in the break in backward-compatibility but
this update to the turbulence modeling is an essential step in the
future of OpenFOAM to allow more models to be added and maintained for a
wider range of cases and physics.  Over the next weeks and months more
turbulence models will be added of single and multiphase flow, more
additional sub-models and further development and testing of existing
models.  I hope this brings benefits to all OpenFOAM users.

Henry G. Weller