fill the positions that describe how the data in the state
vector relates to the entries in the buffer use memory that
is allocated by mpi for the buffer.
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.