Use correct discriminators for climatological vs observational variable

[rod-glover]

Resolves #23

Notes:

• The core fix is a matter of a few lines of code. Much effort was needed to clean up the chaotic code in conftest.py before the testing setup could be updated and the existing tests run.

• Updates pycds to version 4.4.0, which is the first version that contains the discriminator function variable_tags. This update prompted much of the test code cleanup, since database initialization is more complex, and in particular requires applying the database migration to install non-table objects such as functions.

• A likely bug was found and fixed in the test code for the affected handler.

[rod-glover]

@jameshiebert , do you want to glance over this again. Answers to your questions are posted, a few code changes made.