sci-astronomy/casapy-bin: Add ebuild for casapy

Added an ebuild for the tarball distribution of casapy. The newest "release" was not recommended for cycle2 ALMA observation proposals, so I figured it's worth just using these prereleases of 4.2.

There's some issues I'm aware of with this ebuild:
* I don't know how to make the ewarn only show up for selinux profiles
* The dependencies come from running ldd on everything executable, and excluding everything bundled with casa and not in @system. I'm not sure if it's overkill.

I'd appreciate any feedback on this (admittingly simple) ebuild.