ENH Store sensitivity history arrays

[drewejohnson]

Is your feature request related to a problem? Please describe.
When running sensitivity runs using the sens opt history 1 option, arrays are generated that describe the values of the responses using only N latent generations, from one to one less than specified in the sens opt latgen <> setting. These arrays are not currently stored, but their presence is noted [PR #366]

Describe the solution you'd like
We should find a way to store these arrays. The primary use case would be to study and plot responses and/or uncertainties using variable latent generations.

Describe alternatives you've considered
We could store these values in a similar dictionary as an array of responses latgensens["keff"].shape = (N-1, ...) where N is the number of latent generations, and nth subarray latgensens["keff"][n] is the response using only n latent generations. Or a dictionary of dictionaries could work as well latgensens["keff"] = {n: array}

Additional context
Serpent wiki - http://serpent.vtt.fi/mediawiki/index.php/Sensitivity_calculations#Effect_of_number_of_latent_generations
Original forum post - https://ttuki.vtt.fi/serpent/viewtopic.php?f=30&t=3278