New Package: OpenMC (#4790)
New Package: OpenMC (#4790)

* New Package: OpenMC

* comments addressed.

* comments addressed.

* changed 'os.makedirs' to 'mkdirp'

* flake 8 fix.

krafczyk/spack,krafczyk/spack,skosukhin/spack,krafczyk/spack,LLNL/spack,mfherbst/spack,TheTimmy/spack,iulian787/spack,lgarren/spack,skosukhin/spack,EmreAtes/spack,krafczyk/spack,LLNL/spack,iulian787/spack,lgarren/spack,lgarren/spack,TheTimmy/spack,mfherbst/spack,skosukhin/spack,lgarren/spack,TheTimmy/spack,TheTimmy/spack,iulian787/spack,EmreAtes/spack,skosukhin/spack,tmerrick1/spack,TheTimmy/spack,EmreAtes/spack,iulian787/spack,matthiasdiener/spack,tmerrick1/spack,mfherbst/spack,EmreAtes/spack,matthiasdiener/spack,iulian787/spack,krafczyk/spack,mfherbst/spack,LLNL/spack,tmerrick1/spack,LLNL/spack,mfherbst/spack,LLNL/spack,EmreAtes/spack,matthiasdiener/spack,tmerrick1/spack,skosukhin/spack,matthiasdiener/spack,matthiasdiener/spack,tmerrick1/spack,lgarren/spack