adding Parma_SplitPartition
allow verify to accept empty parts

with mixed feelings, it should still be
beneficial to verify meshes in intermediate
partitioning stages
allow SplitPartition to skip mesh modifications

In the case of loading in a subset, we will just
avoid remapping inside parma altogether, by
passing a null mesh pointer
demo subgroup loading in test/fan.cc

the 4-proc test loads and adapts a
2-part mesh on procs 0 and 1.
Merge branch 'master' of https://github.com/SCOREC/core