fieldMapper: Simplification

The patch-specific mapper interfaces, fvPatchFieldMapper and
pointPatchFieldMapper, have been removed as they did not do anything.
Patch mapping constructors and functions now take a basic fieldMapper
reference.

An fvPatchFieldMapper.H header has been provided to aid backwards
compatability so that existing custom boundary conditions continue to
compile.

etc/codeTemplates/dynamicCode/codedFvModelTemplate.C

@@ -25,7 +25,6 @@ License
 
 #include "codedFvModelTemplate.H"
 #include "addToRunTimeSelectionTable.H"
-#include "fvPatchFieldMapper.H"
 #include "volFields.H"
 #include "surfaceFields.H"
 #include "unitConversion.H"