Raise warning when size_index set to non-numeric dimension

[philippjfr]

As suggested in #982 currently you get an uninformative exception when setting the size_index option to a non-numeric dimension. This raises a warning and adds unit tests to ensure the warning is raised.

[jlstevens]

I think you could shorten this a bit and avoid some repetition with something like:

sizes =  self._apply_size_index(element, self.size_index)

This could be implemented in a common base class and would always return sizes, issuing the warning when the dimension type is inappropriate.

[philippjfr]

This could be implemented in a common base class and would always return sizes, issuing the warning when the dimension type is inappropriate.

I've so far refrained from shared baseclasses for ElementPlots because a fair amount of the API is different. I'd be happy to factor it out into the shared compute_sizes function that already exists.

[jlstevens]

I'd be happy to factor it out into the shared compute_sizes function that already exists.

That also makes sense.

[philippjfr]

Unfortunately I didn't manage to factor out much, the warning needs the dimension and element name to be useful and I didn't want to add even more arguments to the compute_sizes function.

[jlstevens]

Ok, nevermind then. Merging.