defect/de-46 -- ADP Bulk Load - prevent agent.parcel_size_code foreign
key constraint error

top/adp.py
* update the sanitise() method so that agent.parcel_size_code value
  defaults to "S" if no valid option is provided

top/adpdaemon.py
* update the set_adps() method so that only one code_header value is
  parsed from the CSV file.  All other attempts to load a non-unique
  code_header will log and error and skip that entry