PXD-342 ⁃ Template TSV required field indicator

[cgmeyer]

Nice to have: an indicator or flag in the template TSV headers that Gen3 data submitters use to prepare their metadata which would mark those headers as required; e.g., NCBI templates add an asterisk to the column names for required properties:
*submitter_id, *type, *links, *required_property1, *req_prop_2, optional_property1, opt_prop_2, etc.