Add required PrepTemplate columns for target gene studies

Adds a check for required columns on the target gene studies PrepTemplate.
Modifies the logic of the StudyPreprocessor so it creates a minimal QIIME-compliant mapping file for running split libraries.

@antgonza this should be the last PR for being able to run split libraries through Qiita,
AFAIK, you can insert a single PrepTemplate at a time, so it will be that one. Also, at this point, nothing is added to the DB, so there is no id to the user...