- make the purpose more explicit, and reduces some work for the
  compiler as well.

- this permits direct storage of a list with additional matcher
  capabilities

- provide wordRes::matcher class for similar behaviour as previously

Renamed the original volRegion -> volFieldValue to clarify the purpose
of this class to process vol fields on a volRegion.

functionObjects: Further simplification and rationalization using the fvMeshFunctionObject base-class

regionFunctionObject: Moved the field/object maintenance functions from fvMeshFunctionObject into regionFunctionObject

functionObjects: Clear the result field from the objectRegistry if the argument fields are not available
This is needed to handle the processing of many time directories, some
of which may have the required fields and some not.

functionObjects: New abstract base-class 'fieldExpression' for simple field expression evaluation functionObjects
Updated and simplified 'div', 'grad' and 'mag' functionObjects by deriving from 'fieldExpression'.
Corrected the handling of cached gradients in 'grad'.

to simplify writing common functionObjects and avoid unnecessary code duplication

  - Avoids the need for the 'OutputFilterFunctionObject' wrapper
  - Time-control for execution and writing is now provided by the
    'timeControlFunctionObject' which instantiates the processing
    'functionObject' and controls its operation.
  - Alternative time-control functionObjects can now be written and
    selected at run-time without the need to compile wrapped version of
    EVERY existing functionObject which would have been required in the
    old structure.
  - The separation of 'execute' and 'write' functions is now formalized in the
    'functionObject' base-class and all derived classes implement the
    two functions.
  - Unnecessary implementations of functions with appropriate defaults
    in the 'functionObject' base-class have been removed reducing
    clutter and simplifying implementation of new functionObjects.
  - The 'coded' 'functionObject' has also been updated, simplified and tested.
  - Further simplification is now possible by creating some general
    intermediate classes derived from 'functionObject'.

Construction failure and recovery is not handled with exceptions in functionObjectList

functionObjects: Moved into the functionObjects namespace and rationalized and simplified failable construction

Rather than requiring each functionObject to handle failed construction
internally (using the active_ flag) the static member function "viable"
is provided which returns true if construction of the functionObject is
likely to be successful.  Failed construction is then handled by the
wrapper-class which constructs the functionObject,
e.g. "OutputFilterFunctionObject".

Updated objects
- corrected Peclet number for compressible cases
- propagated log flag and resultName across objects

New function objects
- new fluxSummary:
  - calculates positive, negative, absolute and net flux across face
    zones
- new runTimeControl
  - abort the calculation when a user-defined metric is achieved.
    Available options include:
    - average value remains unchanged wrt a given threshold
    - equation initial residual exceeds a threshold - useful to abort
      diverging cases
    - equation max iterations exceeds a threshold - useful to abort
      diverging cases
    - min/max of a function object value
    - min time step exceeds a threshold - useful to abort diverging
      cases
- new valueAverage:
  - average singular values from other function objects, e.g. Cd, Cl and
    Cm from the forceCoeffs function object

end of Time::operator++. This allows to know if the next timeIndex will be
a dumping time. The function object "partialWrite" modifyes the write option
of the those fields which will be written down at given intervals of the overall
outout times.

This reverts commit c5bea524.

This reverts commit b18f6cc1.

Created new functionObject for dsmcFields, renamed post processing utility dsmcFields to dsmcFieldsCalc and calling function object from there.

- added end() method to functionObject, functionObjectList & associated classes
- moved outputFilters from src/sampling -> src/OpenFOAM/db/functionObjects