Use pint for units

[domna]

For easier unit conversion and loading we want to use pint. There are a few considerations

• As it integrates nicely with numpy arrays, we should make sure that it also works good with pandas and xarray (to support the further data representation path).
• It should be usable without overhead when default units are used, so maybe conversion can happen automatically for wavelengths in nm.
• It should make using data in other units as easy as possible (also conversion from wavenumber and energy), so it should be not more as using an ureg("eV") to specifiy the unit of the data used.