Add on-the-fly conversion of units

[tcmetzger]

Add on-the-fly conversion of units of measurement (e.g. convert temperatures from C or F to K) to the Python frontend

[brendancol]

@RobertPincus don't mean to be a wet blanket here, but I think sometimes this is better handled by the user and not part of pyrte.

[RobertPincus]

@brendancol We will surely not get to this in Phase 2 but I would leave it as an open idea.