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,
Can you add this test for the case where there's one of the required columns but not the others?