gather the indexed mpi datatype, which describes how the data in the state
vector relates to the entries in the buffer.
in contrast to the simple indexed type above, we try to minimize the number
of blocks here, and gather contiguous blocks of memory together.
Nodes of different colours represent the following:
Solid arrows point from a procedure to one which it calls. Dashed
arrows point from an interface to procedures which implement that interface.
This could include the module procedures in a generic interface or the
implementation in a submodule of an interface in a parent module.