Provide annotation for units of outputs

Currently numerical outputs are in reduced units (or default units). There should some extension to convert units if necessary. Example: vibrational energies can be in eV but sometimes can be more easily compared in wavenumbers (cm**-1).

A first solution is to implemented units converter as an option for the print() statement. This is more or less straight forward. The drawback is that the stored value is possibly in other units and only the displayed value.

An alternative way is to store the values in the suggested units. This might be more involved because the annotation (and converter) must be in the variable statement.