Support Subtract and Division of NDCube by NDData

[DanRyanIrish]

To Do

• Add dask test
• Rework subtraction to avoid in-place modification
• Merge Add NDCube method to convert to NDData instance. #887

[wtbarnes]

I've tried to wrap my head around what's happening here and I don't see anything obviously problematic (though I don't know much about NDData either). My one suggestion would be adding a test that constructs an NDData from a dask array (I think this is possible??) and then apply a these operations (at least add and mul) to that object and ensure that the laziness is preserved in these new codepaths.

[DanRyanIrish]

@wtbarnes: Do these tests satisfy your comment?

• subtraction dask test
• division dask test

[DanRyanIrish]

@Cadair @wtbarnes: Docs on Arithmetic operations are now included in this PR. Please let me know if you have comments. If not, I'd appreciate an approval :)

[Cadair]

Need to decide on what we are doing with the open docs comments.