add science/libgeodecomp - auto-parallelizing library for computer simulations

LibGeoDecomp (Library for Geometric Decomposition codes) is an
auto-parallelizing library for computer simulations. It is specifically targeted
computational science applications in the context of supercomputers and
heterogeneous systems. The library eases development of such tightly coupled
codes by essentially taking over the parallel programming: it handles both, the
parallelization itself and parameter tuning. This enables scientists to focus on
their simulation code, rather than the technical details of the parallel
computer.

PR: 213688
Submitted by: Kurt Kanzenbach <kurt@kmk-computers.de>
Reviewed by: matthew
Approved by: matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D9390