Speed up Python package installation

[rod-glover]

In #199, we changed the Python CI installation (34f391a : python-ci.yaml) to speed up the Python installation. The commit message details what was done and why:

The base image for CI comes with GDAL and NetCDF Python modules pre-installed. We want to use the system Python so that we don't have to reinstall these (expensive to install) modules.

That makes total sense. Unfortunately, the update proved not to be robust. Meaning, with the passage of time, some change in the CI infrastructure and/or pipenv and or ??? meant that the installation began failing, apparently not due to any change we made later.

Investigations showed that the --system option causes the installation failure (now).

Let's try to do this in a different way so that it is more robust.

[rod-glover]

Also, it seems contrary to the normal use of pipenv to install the project without it (pip install . instead of pipenv run pip install . and ditto to run the tests outside of pipenv. Not sure what the thinking was here.

[Nospamas]

Bumping this issue a bit. I'm noticing the docker build takes 15+ minutes, even locally taking 10+minutes not really very responsive.