Allow omitting nature in IV calc

[Lincoln-LM]

This PR serves to allow omitting nature in IV calculation for easier calcing of shadow pokemon's IVs pre-purification

This is a pretty simple change, so if there's a different way you want to go about doing this/any other comments lmk

[Admiral-Fish]

Will this be useful after the redesign branch? I am adding showing stats

[Lincoln-LM]

I'd argue it's less useful with showing + filtering by stats implemented, but unless iv calc is entirely removed its a simple change thats still an improvement allows you to also use it with shadows (or in a scenario where you can only see stats in battle and not nature).