Fixes inhale_pp ratio

The amount of inhaled gas used shouldn't approach infinity as inhale_pp
goes to zero, should it?

Also the amount of gas used should decrease with inhale_pp as the
oxyloss is being caused by the fact that fewer moles are entering the
bloodstream.
More sensible oxyloss scaling
Should fix #702, volume pumps always outputting.
Should fix #727 for humans and monkeys.

Conflicts:
	code/modules/mob/living/carbon/human/life.dm