PVEngine.skip_step is not used

[kandersolar]

PVEngine creates a mask indicating values that need not be considered in the calculation:

https://github.com/SunPower/pvfactors/blob/21d48655ce6acd3ab1955f98212cbdd3e3cc8351/pvfactors/engine.py#L170-L175

However, that mask is not actually used anywhere today. It used to be, but it looks like it stopped being used in #84 during the switch from iterative to vectorized code.

I think the underlying concept may still be useful even in the current vectorized approach, so I think it's probably worth seeing if we can start using it again somehow, otherwise it should just be deleted IMHO.

[anomam]

Thanks for catching this @kanderso-nrel ! Looks like I missed this part when updating things. I'm generally of the opinion of deleting dead code, and we can always find it again in the git history if necessary.