Add python-netcdf4

Read the commit logs. This turns out to be tricky and exposes several deficiencies in current hashstack.
This PR builds, so it shows how to fix the problems.

To make this nice, I suggest in future PRs:

* use `libflags.yaml/py` in autotools as well as cmake scripts in `base/` (i.e. only implement the `CPPFLAGS` functionality once) 
* Fix the issue that hdf5 is not pulling in mpi automatically, so netcdf4 and python-netcdf4 must depend on mpi explicitly