refactor create_qiime_mapping_file

[antgonza]

From @josenavas in #678:
It is a bit weird that we are relying on the filepaths to create the qiime mapping file (so we can parse them into pandas). I remember correctly, there is a function that @ElDeveloper wrote to transform a MetadataTemplate into a pandas DataFrame. I think the way to go is to make a function on the MetadataTemplate object called to_dataframe that returns a DataFrame with the template information.

@antgonza note: this will duplicate some code (as this will be similar to what we are doing now but retrieve the info from DB and generate the pandas dataframe) and then load in memory but is another option.

[ElDeveloper]

As a future reference, the name of the function is dataframe_from_template.

[antgonza]

I think this can be closed as everything here is implemented. Please reopen and add details if that's not the case.