feat(fixtures): add use_pandas pytest fixture and --pandas CLI arg

[wpbonelli]

Tabular data is commonly represented in Python with numpy recarrays and pandas dataframes. FloPy recently added pandas as a core dependency. Work is underway to use pandas internally where expedient and support pandas dataframes across user-facing APIs where numpy recarrays were previously required.

Introduce a --pandas (short -P) pytest CLI option accepting values: yes, no, random. This controls the value of a boolean use_pandas fixture. Motivations:

• allow parametrizing use_pandas argument to MFSimulation etc
• allow controlling how package input data is provided in tests

Other miscellany:

• some minor cleanup/reorg
• use dest="KEEP" when configuring --keep option, access option from fixtures with request.config.option.KEEP