api, base, mix: Enable multiple compound lists

Loading only two lists of rigid layout became is a bit
unflexible. Introducing the option to load instead multiple compound
lists that must conform to a certain format.

* R/api.R(load_compound_input): Updated. Now reads multiple compound
  CSVs, if provided.

* R/mix.R(read_set_id): Updated.

* R/mix.R(verify_cmpd_l): New function.

* R/resources.R(EMPTY_CMPD_LIST): New parameter. The layout of a
  compound list.