Allowing tabulated dispersions to be added properly

[domna]

Problem: Currently tabulated dispersions are allowed to be added together. However, they still keep their interpolated data range so requesting a dielectric function across the two dispersions raises an error. This problem partly applies to the combinations of tabulated data + dispersion function.

Solution: It could either be solved by allowing to interpolate over two tabulated datasets when they are added together or raising an error when trying to add tabular data together. For tabulated data + function I think it is reasonable to just allow the interpolation range as is and raise an error otherwise.

Tasks:

• A cleaner error if the data is outside the interpolation range.
• Tests
• Default wavelength/energy range set to the whole interpolation range. Currently, this raises an error if the interpolation range is not inside the default range.