Add function for concatenating colormaps

[mandli]

The class LinearSegmentedColormap also gains the functions __add__ and
__radd__ as a shortcut to the function.

[mandli]

@dmcdougall What is the status of this PR? Should we assume that we need to maintain direct support for this ability or depend on matplotlib to do this?

[mandli]

FWIW, I had to rework this into a more complex function that returned a normalization object as well.

[mandli]

Merged similar functionality elsewhere.

[mandli]

Ug, sorry, different repos.

This approach to merging colormaps has issues related to the normalization object associated with colormaps. There is a fix but is not quite as nice as simply adding colormaps together. See the add_colormaps function in clawpack for an example.

[dmcdougall]

I think this is specialised enough that a clawpack function is sufficient. Maybe we should ping the matplotlib community to gauge whether or not this functionality would be useful in other domains.

[mandli]

Sounds good.