Derive package build number from number of git commits since last tag

[breathe]

Derive the conda PKG_NUMBER from the number of git commits since the last tag.

You should create a tag somewhere in the repo -- suggest tagging the initial repository commit as '1.1'. The build number will then be the number of commits in the repository since the initial commit -- which is approximately monotonic -- should ensure that libraries which use this package can update to latest build more safely.

When you want to update a project to use updated build of coffee_poisson_solver_ko -- I believe you can just:

conda update coffee_poisson_solver_ko  # update package
conda env export > ...  # keep a shared environment description up to date