Split libraries fastq

Adds the StudyPreprocesser class, which allows to preprocess the raw data of a study. Currently, it only supports Illumina data, but the code should be easy to expand to support other types.

I've also modified the `insert_filepaths` function, so it now supports to pass the filepath type either by id or by the string (name).

I've modified the initialize.sql file, to support the correct filetypes for the Illumina data and its preprocessed data. I had to modify multiple files due to this change.
Do we need a `default_working_dir` here, like we have for the base_data_dir, above?  Note that some of this conflicts/recapitualtes what I also implemented in #366, but it shouldn't matter too much which we merge, since they both accomplish the same thing.