Change the reference scenario (for calculating the BECCS bound, see previous commit) from input_ref.gdx to the values of the own previous nash iteration.

Reintroduce bound on biomass relative to reference scenario to prevent regions with large carbon storage potential from deploying BECCS in tight policy cases only to produce negative emissions rather then generating energy.

bugfix 0935e7ee, reintroduced an if/else condition for a bound on biomass and renamed c_1stgen_phaseout into cm_1stgen_phaseout

*refactoring* Deleted q30_pebiolc_price_base and vm_pebiolc_price_shifted since it was a remainder of old implementations and had no function anymore.