Conda packaging

[davidrich27]

• Adds meta.yaml and build scripts for conda packaging
• Adds github action CI for conda packaging
• Adds GPL license
• Adds versioning that gives git-hash and date of current build.

[marybarker]

Looks fantastic so far! Some conversation points we want to check off:

• change the name it's published under on conda to larch-phylo so that it can be published on conda-forge. (There's already a conda-forge::larch package)
• change the other executables dagutil, dag2dot, etc. to larch-dagutil, larch-dag2dot, etc.
• correct license?
• possibly make a working version for MacOS as well.