Potentially remove support for functions that accept numpy arrays rather than DataArrays

[spencerahill]

(Another potential piece of fat to be trimmed from Calc found while working on #208)

Calc currently has logic for handling when a Var's function consumes numpy arrays rather than xarray.DataArrays. See here. IIRC, the motivation for this was to ease the transition from when we first switched over to a DataArray-based workflow, so that existing functions that used numpy arrays needn't be scrapped. The flag itself is actually an attribute of Var.

However, I have already switched over everything to DataArrays. Does anybody use this? @spencerkclark?

Compared to much of Calc, the logic is actually straightforward, but I still think it fits into the category of non-essential, poorly or not-at-all documented things that I crammed into Calc that would be better off re-implemented properly if/when the need arises. That being said, 100% receptive to keeping it if it's still being used.

(There is also placeholder logic for functions that consume Datasets rather than DataArrays, which likewise seem unnecessary.)

[spencerkclark]

Yes, by all means remove this! This is quite outdated; I don't use it either. Regardless, with apply_ufunc now in xarray, it is quite straightforward to convert a pure numpy function to one that operates on DataArrays.

[spencerahill]

Regardless, with apply_ufunc now in xarray, it is quite straightforward to convert a pure numpy function to one that operates on DataArrays.

Great point. OK, will go ahead with this.