Add crest program to package index

[awvwgk]

This patch adds the conformer-rotamer ensemble search tool (crest) to the package index.

• OpenMP based task scheduler and automation tool for exploring potential energy surfaces written in Fortran
• the package supports meson, CMake and plain make as build systems
• compiles with Intel Fortran and GFortran, requires MKL
• documentation can be found here: https://xtb-docs.readthedocs.io/en/latest/crest.html

cc @pprcht

[certik]

Looks good to me!