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,
I would expect this to require the linkerprimersequence but I guess there's something I am missing, can you explain this?