Move stitching code to prediction module

[davidwilby]

@MartinSJRogers - Tom and I are keen to move some of the patch stitching to the pred module from model since it operates on predictions.

Tests are passing but could you take a look a this please? In particular some of the docstrings and the descriptions of X_t to advise me of any changes as I'm not sure these make sense.

While we're at it, the docstring of _get_coordinate_extent I'm not really sure I follow:

Get coordinate extent of dataset. This method is applied to either X_t or patchwise predictions.

Could you suggest something more descriptive please?

Finally, should we also consider moving get_patch_overlap to the same module? And possibly moving its call inside stitch_clipped_predictions?

[MartinSJRogers]

Approved- a few minor amendments to mark up text found in my PR: https://github.com/davidwilby/deepsensor/tree/mr_move_stitching_to_pred