ENH: simplify checks and init code with available functions
ENH: simplify checks and init code with available functions

- Creation of interpolators more DRY now
- Disallow anything but None as nn_variants entry for dimensions not using
  nearest neighbor in PerAxisInterpolation
- Simplified check for sizes and shapes in _Interpolator init

odlgroup/odl,aringh/odl,aringh/odl,kohr-h/odl,odlgroup/odl,kohr-h/odl