Add feature to change # of lifetimes over the course of an optimization run

[WillMorrison]

Add a --starter_number flag for the number in the first generation, increasing linearly to the final --number by the end of the optimization.

[WillMorrison]

Turns out this is hard (within the pyeasyga framework). What would be easier (and perhaps even still valuable) is to just change the number of lifetimes simulated in the final validation run