type check in ArgumentParser.Done() or before

[jsc86]

If the content of a presumed command line argument does not fit the value type of assumed argument specification, there is not explicit type checking at all. Access fails late by implicit type checking, when the value is accessed (via ParserResult or rule checks) or mapped (ParserResultMapper/ParserResultConfigBackend). It should fail early and/or handled gracefully by not assuming it to be the expected argument in CommandLineParser.Parse. CommandLineParser.Match/Argument.Done would likely be the right place for graceful handling.