Operator construction, application rewrite

[a-alveyblanc]

Refactors op.py so that operator application is more unified and adds an explicit file for constructing operators. Adds support for sum factorization with tensor-product discretizations. reference_mass_matrix, reference_face_mass_matrix, and reference_inverse_mass_matrix are marked as deprecated since they are public methods (nothing in grudge uses them anymore, but I thought this was the safest option for compatibility).

• Moves operator construction to new file bilinear_forms.py and forms all operators using quadrature
• Introduces support for tensor-product sum factorization with appropriate escape hatch
• Refactors op.py around a general operator application routine
  • Applies dense operators as a matvec or tensor-product factors using tensor contractions over appropriate axes
  • Metric/area terms are applied before/after the the general application routine acts on input DOFs depending on the operator
• weak_local_grad and weak_local_div use weak_local_d_dx to compute their respective results

[inducer]

I like the direction. Could you investigate the failures?

[a-alveyblanc]

Patched example/test failures. Remaining basedpyright failure is due to warnings. I'll address those sometime this week.

[a-alveyblanc]

@inducer Ready for a look. basedpyright failure is due to warnings that come from weird typing interactions between libraries as we discussed.