DDT support for MPI_BOTTOM with derived datatypes
This change adds support for using MPI_BOTTOM in AMPI communication routines: https://charm.cs.illinois.edu/gerrit/#/c/1336/
The test in tests/ampi/mpich-test/pt2pt/sendrecv4 now passes for predefined types and derived types that are composed of predefined types, but fails on the first case that is a derived type composed of other derived types. If you use gdb on sendrecv4 you can see that it fails when handling a type that is a struct type with a vector type in it.
I suspect that DDT lacks support for such types in more ways than for MPI_BOTTOM (lower bounds)