Add a conversions module to express common "equivalencies" that are actually in non-equivalent units

[matterhorn103]

astropy allows the use of Quantity.to() for conversions that don't really fulfil dimensional analysis, such as expressing temperatures as energies in eV via multiplication by the Boltzmann constant.

quanstants won't support the same API, but it is probably worth providing a set of functions that do the same thing.