meshToMesh, mapFieldsPar: Rationalisation

Cell-to-cell interpolation has been moved to a hierarchy separate from
meshToMesh, called cellsToCells. The meshToMesh class is now a
combination of a cellsToCells object and multiple patchToPatch objects.
This means that when only cell-to-cell interpolation is needed a basic
cellsToCells object can be selected.

Cell-to-cell and vol-field-to-vol-field interpolation now has two well
defined sets of functions, with a clear distinction in how weights that
do not sum to unity are handled. Non-unity weights are either
normalised, or a left-over field is provided with which to complete the
weighted sum.

The left-over approach is now consistently applied in mapFieldsPar,
across both the internal and patch fields, if mapping onto an existing
field in the target case. Warning are now generated for invalid
combinations of settings, such as mapping between inconsistent meshes
without a pre-existing target field.

All mapping functions now take fields as const references and return tmp
fields. This avoids the pattern in which non-const fields are provided
which relate to the source, and at some point in the function transfer
to the target. This pattern is difficult to reason about and does not
provide any actual computational advantage, as the fields invariably get
re-allocated as part of the process anyway.

MeshToMesh no longer stores the cutting patches. The set of cutting
patches is not needed anywhere except at the point of mapping a field,
so it is now supplied to the mapping functions as an argument.

The meshToMesh topology changer no longer supports cutting patch
information. This did not previously work. Cutting patches either get
generated as calculated, or they require a pre-existing field to specify
their boundary condition. Neither of these options is suitable for a
run-time mesh change.

More code has been shared with patchToPatch, reducing duplication.

tutorials/modules/CHT/heatExchanger/constant/air/fvModels

@@ -21,7 +21,7 @@ airToporous
     nbrRegion       porous;
     master          false;
 
-    interpolationMethod cellVolumeWeight;
+    interpolationMethod intersection;
 
     semiImplicit    no;
 }
@@ -34,7 +34,7 @@ porosityBlockage
     {
         nbrRegion       porous;
 
-        interpolationMethod cellVolumeWeight;
+        interpolationMethod intersection;
 
         type            DarcyForchheimer;
 

tutorials/modules/CHT/heatExchanger/constant/porous/fvModels

@@ -21,7 +21,7 @@ porousToair
     nbrRegion       air;
     master          true;
 
-    interpolationMethod cellVolumeWeight;
+    interpolationMethod intersection;
 
     semiImplicit    no;