BUG: subsetMesh: map instead of truncate. See #1558.

This also is to do with redistributePar:
this uses subsetMesh to generate parts to
send to different processors.
2) related to 1558: make sure not to choose 'mapped'
patches to move the processor patches into so
we can use the mapper cloning and correctly
size additional data (e.g. offsets). This should
be generalised to hold for any patch type
holding local data ...