Update to mass labels and possibility to use interpolated SSPs

Firefly second release, with two main updates:
i) The labels for the masses have been updated so now the Stellar Mass only refers to the living stars and remnants, but not the gas. There is now a label total_mass for the mass including both the stellar and gas component.
ii) The code has been updated to read new SSPs obtained interpolating in metallicity from the previous ones.