Specialize ExperimentAxisQuery.{obs,var} to pa.IntegerArray

[ryan-williams]

Issue

• SparseNDArray.read requires a SparseNDCoords arg. This can be a tuple of pa.IntegerArrays, but not a tuple of pa.Arrays.
• We nevertheless pass pa.Arrays in two places:
  • ExperimentAxisQuery.X passes self._joinids.{obs,var}; both are pa.Arrays.
  • _fast_csr.read_csr passes {obs,var}_joinids; both are pa.Arrays.
• In reality, the values are actually pa.IntegerArrays, so it's valid at runtime, but the types don't reflect that.
• IntelliJ was warning me about it; not sure why mypy wasn't catching it.

Fix

• To address this, ExperimentAxisQuery.{obs_var}_joinids (and some related fields, e.g. _JoinIDCache._cached_{obs,var}) should be pa.IntegerArrays, instead of pa.Arrays.
• Narrowing those return types is backwards-compatible, afaict.

[johnkerl]

LGTM!

[bkmartinjr]

LGTM