Consolidate axis conversion codes

[jlaehne]

• As pointed out in Fix bugs in to_raman_shift #140 (comment), there is quite some duplicate code in the if/else statements of the axis conversion codes that can be consolidated.

• It would be desirable to allow any cross-conversions between wavelength, wavenumber and energy (documenting in the metadata the conversions to warn if multiple conversions are done on the same axis). To this end, it may be considered to move the main part of the code into a generalized conversion function.