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,
Worth noting that while this is a minimal mapping file the information written in the `Description` column is spurious i. e. it is and will always be `NA`, doesn't matter if the original `prep_template` included a `Description` column.