Use a proper expression parser for unit conversions

[Luthaf]

Currently, only units explicitly defined in the code are supported for models inputs and outputs. This is starting to be problematic, and will become more and more problematic as we increase the number of supported outputs and supported units.

We should replace the current unit_conversion_factor(quantity, from_unit, to_unit) function with just unit_conversion_factor(from_unit, to_unit). The function would then parse the unit expressions to make sure both units have the same dimensionality, and return the conversion factor between them.

I have an example of unit parsing code here https://github.com/lumol-org/lumol/blob/master/lumol-core/src/units.rs that we could re-use pretty easily.