LAMA to Dask: arithmetic, logical & comparison operators

[sadielbartholomew]

Convert all of the arithmetic, logical & comparison operators from LAMA to Dask. Please cross-reference with #295 for the precise listing of methods being migrated here, if necessary.

[davidhassell]

Thanks, Sadie. A quick review, as I know that there is more work to done ...

notably since I am about to add a commit or two applying elemwise calls to the operations that give the results

I don't really understand what these elemwise calls could be for - am I missing something?

[sadielbartholomew]

Thanks for all of your feedback (so far) @davidhassell, which I have now addressed fully as far as I can tell.

Unless you have further comments, this should be ready to merge now, though I would like to also first check that I was OK to 'deprecate' our custom function _numpy_isclose by just removing it, rather than removing the functional code inside it and adding a deprecation warning, due to it being an internal function?

[sadielbartholomew]

(CI jobs failing or cancelled due to trying to use the wrong cfdm version as a dependency, which I will fix in the workflow but outside of this PR; overall we don't have to worry about it here. Locally the relevant test, test_Data.py, passes for me.)

[davidhassell]

Hi Sadie - all looks good to me, thanks. As _numpy_isclose is an underscore method, I'd be fine with just deleting it. I've confirmed that the units tests pass on my laptop. Please merge!