Fix bug in Memory reader
CSV writer now warns about implicit conversions
Add IntegrationHelper module for testing

- add io_test class method for setting read_type and write_type
  attributes for all combinations of readers and writers
- add get_reader and get_writer methods for instantiating readers and
  and writers
- add convert_rows method for converting data forms with no other
  changes
- add migrate method for testing result of migration between two data
  types