added fixed_bsm

[FrancescoMerlotti]

The aim of the PR is being able to perform PDF fits with a fixed BSM Wilson coefficient or Wilson coefficients linear combinations, using the simultaneous fit framework setting the simu_parameters to a fixed value using the constant keyword initialisation and adding fixed_bsm: True to the n3fit runcard.

• Added fixed_bsm parameter to performfit function in n3fit/src/n3fit/performfit.py
• Added fixed_bsm parameter to ModelTrainer object initialisation in n3fit/src/n3fit/performfit.py
• Set CombineCfacLayer object to have untrainable weights if fixed_bsm is True in _generate_observables method of ModelTrainer class in n3fit/src/n3fit/model_trainer.py

I will add an example runcard once I test everything works properly