FIX avoid registering the same colormap twice to matplotlib

[TomDLT]

Pycortex raises an error with matplotlib 3.6, when it tries to register the same colormap twice. This is because the check if self.cmap not in cm.__dict__ does not work anymore. (See recent changes to the colormaps API in matplotlib here)

This PR:

• fixes how to check that the colormap is already registered
• refactors duplicated code into a new method get_cmapdict in Dataview and DataviewRGB
• add a test for clarity

I tested the fix with matplotlib 3.3 and 3.6.

[mvdoc]

Thanks!