[Caching] Deprecate `where`

See merge request inducer/pytential!171

# Conflicts:
#   pytential/qbx/__init__.py
#   pytential/symbolic/primitives.py

[Caching] Interpolation Mapper

See merge request inducer/pytential!172

This should make the tests pass (hopefully). Changes
* made `map_elementwise_max` and friends do the subsampling, since it
does not make sense for a generic vector to be subsampled like it was.
* made sure everyone was tagged correctly for this to work.

The mapper looks for a num_reference_derivative on `quad_stage2` and
replaces it by one on `stage2` wrapped inside an interpolation to
`quad_stage2`.

Delete unused map_quad_kernel_op

See merge request inducer/pytential!165