Better naming convention for simulation based versus closed form solution

[andytimm]

Using retrodesign vs. retro_design for the simulation based versus closed-form version of the core function is just... not a good idea. It should be clearer which is which, not sure what I was thinking at the time other than wanting to keep retrodesign as the simulation based solution name because that's what it is in the original papers.

I will probably preserve retrodesign's name and soft-deprecate retro_design by pointing it to a clearer name like retrodesign_closedform (will think on the final name).