Mapping with interpolation now behaves correctly when a single cell maps
to multiple faces. In addition, the mapping structure is more compact
and no longer results in copies being made of entire internal fields.
This has been achieved by rewriting the mapping strategy in
mappedPatchBase so that it maps from a reduced set of sampling locations
to the patch faces, rather than directly from the cells/faces to the
patch faces. This is more efficient, but it also permits multiple
sampling locations to be sent to a single cell/face. Values can then be
interpolated to these points before being sent back to the patch faces.
Previously a single cell/face could only be sent a single location onto
which to interpolate; typically that of the first associated patch face.
The resulting interpolated value was then sent back to all associated
patch faces. This meant that some (potentially most) patch faces did
receive a value interpolated to the correct position.
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