feature: specify different frequency dependent factor file for each BS section

This merge request adds the possibility to specify a different frequency dependent factor file for each BS section, which is needed for the HL-LHC model. The file is specified in the settings of the beam screen sections as an additional field in the settings dictionary and it can be either a file in the lhc_pywit_model/lhc_elements folder (there is only one file for the moment but a new one will be with the upcoming HL-LHC merge request) or any user specified file.