Flight 4.0.0

Fixes

• Fix erroneous check in ArgParser whereby there would always be an IllegalArgumentException for the error cause.
  • The logic behind setting the exception cause could never yield true as the requirements were impossible to meet within the enclosing if block. This logic has been simplified to only check for the existence of a String error message returned from the rangeCheck function instead.

Changes

• Remove the need to wrap returned values in parsers with Optional.
  • Instead, you now return the parameter directly, or null, rather than e.g. Optional.ofNullable(value).