Regselect: catch errors in refit()

[shanemcq18]

• Catch any errors in fit_regselect*() that occur when the model uses refit().
• Tikhonov-type least-squares solvers do not require the regularizer in the constructor but will raise an AttributeError in solve() (and other methods) if the regularizer is not set. This makes using fit_regselect_*() much less cumbersome.
• PODBasis.fit(Q) raises a warning when using the "method-of-snapshots"/"eigh" strategy if $n < k$ for $\mathbf{Q}\in\mathbb{R}^{n \times k}.$ In this case, calculating the $n \times k$ SVD is likely more efficient than the $k \times k$ eigenvalue problem.
• Added Python 3.13 to list of tests.
• Added one reference to the literature page.

Version update 0.5.13 --> 0.5.14.