Add the option to *not* train the preprocessing

Created by: scarlehoff

This PR adds the possibility for setting the training of the preprocessing on and off for each flavour directly in the runcard.

  basis:
      - { fl: sng, smallx: [1.05,1.19], largex: [1.47,2.70], trainable: False }
      - { fl: g, smallx: [0.94,1.25], largex: [0.11,5.87], trainable: True }