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,
Note that 'Description' is a required column on the SampleTemplate, so based on #388 it should never be on the PrepTemplate. That issue is resulted from the code used for the metadata summary that you wrote (merging the two DataFrames) and it also makes perfect sense to not replicate data.