Add unit test for evolution of SSP color with age

[irinaene]

Add unit test for evolution of SSP color with age for MIST, BaSTI, and Padova isochrones.

[bd-j]

Thanks for this! Can you replace some of the explicit code with loops over e.g. ['B','V', 'R', 'I', 'J', 'K'], and keep everythin in an nage x nband shaped array? Also you can use a list comprehension for the loop over age. Makes everything more concise, and easier to augment later etc. cc @dweisz