Reads and writes the field of a member of a CsvRecordAttribute type that has a CsvConverterAttribute, for types that cannot be read otherwise and for formats of your own.
A converter needs a public or internal parameterless constructor. Each mapper and record writer creates one instance and calls it for every record; the workers of CsvReader.ParallelEnumerate<T>(int) share a mapper, so a converter must be thread-safe (one without fields is).