More tools for fractional designs

[dreavjr]

Hi! I added two more tools to deal with fractional designs:

fracfact_opt => scans several generator strings until it finds the one with less aliasing fracfact_aliasing => lists all aliases in human-readable format

I hope you'll find those useful. I created the functions to solve an actual design of an experiment I'll perform, so I believe it might be handy for other researches doing large experiments.

I did some experiments, playing around with alternatives that considered or not the minus signs in the generator strings, but after some experimentation and some math, I concluded that adding signs slowed the optimization quite a bit, without adding any extra benefit.