Add basic support for bidirectional spectra

For #84 (closed)

It went rather smoothly thanks to the previous refactorings.

Since all the reading now happens in read_from_input_information (!149 (merged)), I only needed to do the change there: there is now a specific reading of the energy and spectra if input_information.bidirectional is True.

All the rest of the MR is just adding the relevant getters/setters here and there for the GUI to work properly.