Bugfix shape masking

[hsteptoe]

🚀 Pull Request

Description

Fixes #6126 by exposing the shape geometry to the user. Adds to docs and docstrings to make it clearer how to use this.

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[pp-mo]

From @SciTools/peloton : discussed what we are seeing here and in #6126.
We think this is looking useful, but the two of you seem to have it covered @hsteptoe @acchamber ?
Please shout if you need help.

[hsteptoe]

Discussion with @acchamber notes:

• Don't move imports out of functions as this creates circular import errors
• Check target_crs assignment for occasion when cube doesn't have a CRS
• Re-write conversion of 0-360 -> -180-180 coords in case of degree based CRSs

[hsteptoe]

What are @SciTools/peloton (@pp-mo) thoughts about adding rasterio as a Iris dependency?

This would facilitate better handling of shapes to rasters (which is essentially the problem we're solving) with the added bonus that we could also mask to other shape types, like lines, to extract a trajectory from a Cube, for example.