PriorSet -> PriorDict
BBHPriorSet -> BBHPriorDict
BNSPriorSet -> BNSPriorDict
CalibrationPriorSet -> CalibrationPriorDict

Introduced backports with deprecation warnings for the old classes

1) Rename all tests to the pytest standard naming convention
2) Update to use pytest to run the tests and collect coverage
3) Simplify gitlab CI script using above changes

This reverts commit 46b9e61a.

Note - also forces all `__init__` calls in `prior.py` to use key word
arguments to ease changes in the future.

This reverts merge request !171