Sam fixed len cols

See https://github.com/CommBank/maestro/blob/master/maestro-example/src/test/scala/au/com/cba/omnia/maestro/example/CustomerSpec.scala and https://github.com/CommBank/maestro/blob/master/maestro-test/src/main/scala/au/com/cba/omnia/maestro/test/Arbitraries.scala