Errors propagation

[rugfri]

Somehow related to #94, I was wondering if you think it could be possible to implement the error propagation (on fitting parameters, experimental uncertainties if present/enabled) to the calculated quantities like dielectric function, refractive index, and such. I recently started using https://uncertainties-python-package.readthedocs.io/en/latest/, https://github.com/lebigot/uncertainties/ which may be useful, though I am not sure how far complex numbers/arrays are supported.

Thanks