Propagated unpadded coords in pad

[TomNicholas]

• Closes [BUG] Time coordinate being dropped by apply_as_grid_ufunc #575
• Tests added
• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

[TomNicholas]

@jbusecke to fix #575 we need to change _strip_all_coords to instead strip only the coords which are a function of the padded dimensions. Therefore we need a function like

def _strip_axis_coords(da, padding_axes):
   ...

For _pad_basic this is fine, but for face-connected grids its the implementation is a bit more complicated because pad_axes is not known until after you know you have a face-connected grid.

Instead how about we move the coordinate stripping inside of _pad_basic?

[jbusecke]

For _pad_basic this is fine, but for face-connected grids its the implementation is a bit more complicated because pad_axes is not known until after you know you have a face-connected grid.

I am not sure I follow this entirely. But could we do something like this:

def _strip_all_axis_coords(da, grid):
    # get rid of any dim that is associated with a grid axis
   ...

If I understand this correctly, the problem was not with dims associated with an axis that is not being padded, but instead with dims that are not associated with any axis? This approach would strip more than is needed, but would still probably work?