Implement "classic PET" again in Ladybug

[MatteoFBC]

Hi, that's a proposition to re-add the "PET classic" in Ladybug, alongside the corrected version proposed in https://www-sciencedirect-com.extranet.enpc.fr/science/article/pii/S0360132318301896, to allow model comparison. That implyes to fix some bugs of PET in Legacy related to the reference environment and to the way in which the human subject’s metabolic rate in met was translated into Watts above basal metabolism.
Many thanks!