Build XASObject in GUI via read_from_input_information

Fix #88 (closed)

Thanks to previous refactorings, I was able to completely replace the custom loading code in XASObjectDialog by a simple call to read_from_input_information.

The building of XASObject is now consistent between the GUI and the Ewoks task: both use read_from_input_information. The only difference is that the GUI generates its own input_information by reading the QtWidget values while the Ewoks task takes it as input (if I dare say).