Regselect: accept solver without explicitly setting regularization beforehand

[shanemcq18]

• Improvement to fit_regselect_*() so that the regularization does not have to be initialized before fitting the model. This fixes a longstanding chicken/egg problem and makes using fit_regselect_*() much less cumbersome.
• Ensured error computation correctly aligns training and predicted states in fit_regselect_continuous(). Previously, this could have been misaligned when using a time derivative estimator that truncates states.
• Time derivative estimators in opinf.ddt now have a mask() method that map states to the estimation grid.
• Added regression tests based on the tutorial notebooks.
• Added pytest and pytest-cov to the dependencies for developers.

This also bumps the version to 0.5.15.