ENH: interpolate: uniform xi input arg handling for griddata and interpn
TST: interpolate: add tests for interpn with 1-D xi and xi broadcasting
ENH: interpolate: allow nonscalar-valued values in RegularGridInterpolator
TST: interpolate: reduce code duplication in regulargridinterpolator tests
BUG: interpolate: fix up and test passing in lists to interpn/RegularGridInterpolator
TST: interpolate: test that complex-valued data is handled by rectangulargridinterpolator + interpn