Set the fieldNames that you expect to see in parsed Maps. Deprecated; Parsers should not, in general, be
expected to know what fields they will return. Some individual types of parsers do need to know (like a TSV
parser) and those parsers have their own way of setting field names.
Returns the fieldNames that we expect to see in parsed Maps, if known, or null otherwise. Deprecated; Parsers
should not, in general, be expected to know what fields they will return.