Nicer way of handling default qc_names

[wmacnair]

• consider providing functions default_qcs and all/available_qcs to return lists of QC metric sets, and use default_qcs as the default value for qc_names
• describe all _qcs() getters in one .Rd file, summarizing what each QC is, and which set each function returns.

[wmacnair]

Closed in 310efcd