ValueType.isParsable should be removed

[eeverman]

ValueType.isParsable is never used except by tests and there is no good reason to call it. Directly calling parse and getting the error gives the information that is needed to display to the user, whereas the isParsable method doesn't explain why its not parsable.

Related: Since nulls are handled either by a default value or a non-null constraint, the `ValueType.parse' method should clearly state that it does not expect nulls. Its OK for it to continue to check for nulls and the appropriate response would be null.

[eeverman]

Fixed by #705