Early interp

[inducer]

@alexfikl: @ShawnL00 noticed this. A good place to start is the docstring of the test, to understand the issue being addressed.

Draft because I think EarlyInterpolationAdder may need tweaking to handle all the densities that get thrown at it.

To do:

• Why are we calling weights_area_elements in the iteration loop?
  • get_flat_strengths_from_densities: better cache waa
• How does main avoid upsampling quad weights?
  • Possible: it doesn't. It just does so consistently.
• Add a never-upsample-weights check
• Longer-term: make interpolation need-driven rather than unconditional

Notes:

• This breaks the scalar_int_eq tests because the "input side" of the "Bremer" weights gets processed differently from the output side.
• EarlyInterpolationAdder will happily upsample QWeight, which it very much should not.