PVEngine.skip_step is not used

[kandersolar]

Originally posted in SunPower/pvfactors#139:

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

solarfactors/pvfactors/engine.py

Lines 170 to 175 in 21d4865

	# Skip timesteps when:
	# - solar zenith > 90, ie the sun is down
	# - DNI or DHI is negative, which does not make sense
	# - DNI and DHI are both zero
	self.skip_step = (solar_zenith > 90) | (DNI < 0) | (DHI < 0) \
	| ((DNI == 0) & (DHI == 0))

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.