vector as argument default

[HelenaLC]

• functions can have a vector as an argument default, e.g.,
  qc_names = c("a", "b")
• 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]

Moved default_qcs suggestion to its own issue, because

• it's harder to fix
• I first need to decide how much effort to put into the function calculating QC metrics