Multiplex spectral library

[GeorgWa]

Allow automated multiplexing of spectral library from the config.yaml using a config:

# perform non-isobaric multiplexing of any input library
library_multiplexing:
  # if true, the library is multiplexed
  enabled: False

  # define channels by their name and how modifications should be translated from the input library to the multiplexed library
  multiplex_mapping:
    0:
      mTRAQ@K: mTRAQ@K
      mTRAQ@Any_N-term: mTRAQ@Any_N-term

    4:
      mTRAQ@K: mTRAQ:13C(3)15N(1)@K
      mTRAQ@Any_N-term: mTRAQ:13C(3)15N(1)@Any_N-term

    8:
      mTRAQ@K: mTRAQ:13C(6)15N(2)@K
      mTRAQ@Any_N-term: mTRAQ:13C(6)15N(2)@Any_N-term

    12:
      mTRAQ@K: mTRAQ:d12@K
      mTRAQ@Any_N-term: mTRAQ:d12@Any_N-term

[machth]

Changes look resonable.
Name of parameter is also descriptive in my opinion.
Question would be if you want to set it as dependency of multiplexing parameter, since if this is true you probably usually want to enable library_multiplexing.

[ammarcsj]

LGTM