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.
The paths stored on the database are absolute, but not given the join 148-150, is due to an issue on the insert_filepaths function. I've created a new issue #373, since it is unrelated to this PR and it affects multiple parts around the code.