Refactor `FoamBC` to include BCs set by postprocessors and variables

[mattfalcone1997]

Description

Allow OpenFOAM boundary conditions to be set using postprocessors to facilitate multiscale modelling

Context

Multiscale modelling requires fields to be accumulated into a single value and requires fields to be set by a sinlge value. It is therefore convenient for FoamBCs to be set directly from a postprocessor.

Outputs

• FoamPostprocessorBC base class
• FoamVariableBC base class
• FoamPostprocessorFixedValueBC, FoamPostprocessorFixedGradientBC inherited from FoamPostprocessorBC
• FoamVariableFixedValueBC, FoamVariableFixedGradientBC, inherited from FoamVariableBC replacing existing classes
• New classes for specific BCs: fixed mass flow rate inlet.