Add option to specify minimum message size for unconditional execution of communication to be #processess

[huttered40]

Execution of communication routines following interception by Critter may be skipped altogether by setting two environment variables:

1. one switch telling Critter to do so
2. a minimum message size such that the corresponding communication event cannot be skipped (because it is necessary for the parallel schedule to achieve forward progress).

One (yet-to-be-implemented) option is to tell Critter that any message of size <= np, where np is the size of MPI_COMM_WORLD, must be executed even if the first switch is set to OFF.