Source energy spectrum (#378)

[alexis-pereda]

Replace spectrum_lines source energy type by the three cases that existed in GATE 9, under spectrum. Adds spectrum_type field with three possible values:

• discrete (old spectrum_lines);
• histogram;
• interpolated. See GATE 9 documentation for now (GATE 10 documentation in progress).

Associated test: test010_generic_source_energy_spectrum.py.