mods for Mac distribution

[loriab]

This isn't necessarily in final form even for Mac-only (since it takes so long to test internal build, conda pkg build, detected psi4 build), but I thought I'd let you start looking at it.

• [x] installs python package to site-packages, installs python tests within
• [x] allows simultaneous build of static and dynamic libs closely modeled on https://github.com/SebWouters/CheMPS2/blob/master/CheMPS2/CMakeLists.txt
• [x] changes cmake variable ENABLE_STATIC to STATIC_ONLY for clarity, in my opinion
• [x] what license are you going with? GPLv2+?
• [x] ready to go?

[jturney]

Yeah, I was going with GPLv2. That's what Psi4 uses correct?

[loriab]

Thanks, Yes, Psi4 does GPLv2+. This PR is ready to go now. For now, the Mac psi4 binary will have ambit built in, but the Linux one will not (because I can't face another round of trying to do the static and old libc mods).