Validator for DataArray shapes

[vhirtham]

Currently, we are using the wx_shape validator to ensure that certain dimensions have a specific size. For example, in the SpatialData scheme we want the last dimension that represents space (by definition) to always have the size 3.

This requires us to reorder the internal data of a DataArray during serialization or before to meet the requirements of the validator. This somehow ignores the benefit of a DataArray that all dimensions are named and directly accessible. So we should either expand the functionality of the wx_shape validator or (better I guess) introduce a new one that can handle dimension names too. The syntax can be close to the wx_shape validator just with added dimension names like this: [..., c=3]. This would express "any number of dimensions with arbitrary shape, but the dimension c must have shape (3,)". This would also mean that there has to be a dimension c. [..., (c=3)] would mean that the dimension c is optional, but if present, it has to be of size 3.

[CagtayFabry]

I think implementing this should be really simple if we do it just for DataArray with a mapping like

wx_array:
  c: 3
  n: [5,~]
  time: [2,20]

In the end we should probably just find a nice serialization format to use for our xr_check_coords function inputs (which we basically already have)