Support dask arrays in binary operations

[rcomer]

🚀 Pull Request

Description

Currently, the various arithmetic functions state that their second operand may be a cube, a coordinate, a numpy array or a number. There is nothing to stop you passing a dask array as the second operand, but it will be silently realised by the numpy.asanyarray call, which seems unhelpful to me. I think it would be good to explicitly support passing a dask array, though there may be a better way to do it than my current change.

~In draft for now because a bunch of docstrings would need modifying and some tests adding, but I wanted to see if there is agreement in principle first.~

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[rcomer]

Since nobody has said otherwise, I went ahead and added a test and updated the docstrings. While I was in the arithmetic docstrings, I updated them to make them more consistent with each other and also applied the numpydoc style. Probably the descriptions could be further improved.

Ready for review I think.