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.
We typically format the table names into the queries.  I'm not 100% sure why we do this (I suppose so that if the table names change, we have to change them in as few places as possible?)

If there's a good reason not to do it here, that's fine, but I'm just not fully aware of the convention.