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.
All the split_libraries_fastq.py parameters are supported through the `params` parameters. All possible parameters are stored on a table in the DB, so through that parameter (which has to be a `PreprocessedIlluminaParams`) will ensure that only allowed parameters are passed.