wording in docstring
FatRecord.remove now removes fields or subfields

* if a control field is specified, the controlfield is removed
* if a data field is specified without subfields, all data fields
  with the specified tag are removed, e.g. all fields, that have a
  '020' tag
* if a field.subfield combo is specfied, delete the subfield entry
  and keep the field intact (except the field would be left without
  any subfields, in that case, delete the field as a whole)
new alien