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.
There is a function on IPython to do that, and I think is a decorator for the imports. However, and I remember discussing this with @wasade, we decided that doing it as we are doing here (and other places in the code) is the best route to go. @wasade can you weight?