Add lang/OpenCoarrays, an implementation of Fortran coarrays using MPI.

OpenCoarrays is an open-source software project that supports the coarray
Fortran (CAF) parallel programming features of the Fortran 2008 standard
and several features proposed for Fortran 2015.

WWW: http://www.opencoarrays.org/

PR: 217283
Submitted by: Anton Shterenlikht <mexas@bris.ac.uk>