Add element.where method support dim expressions

[jonmmease]

It would be nice to have an xarray-style where method (http://xarray.pydata.org/en/stable/generated/xarray.DataArray.where.html) on elements.

The signature would be something like

Element.where(cond, other=nan)

Here cond could be a boolean array matching the size of the element, or it could be a dim-expression. The result would be an element of the same type as the input, with the same size, but the values where cond is True would be replaced by other.

One application of this method would be to support arbitrary dim expression filtering on gridded datasets. If other is nan, this would make it possible to overlay a subset of an image on top of itself, where the subset is not a clean slice, but an arbitrary function of the element's dimensions.

[philippjfr]

Similar to #4312, I'll close this and continue discussion there but we may want to keep the where API.

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