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,
Would this be the error that the end user see? I am trying to think of more information that should be added to this error message that would make it straightforward to identify the problematic prep_template. I'll defer to you, because I cannot come up with something.