ENH: Save log file from split_libraries_fastq.py

The log file generated by `split_libraries_fastq.py` was not being stored in
the database. This required three changes:

    - A new filepath_type entry in initialize.sql
    - Modifying `StudyPreprocessor` to be aware of the existence of this file.
    - Updating the tests to account for the points above.

Thanks @josenavas for the help on putting this together.