Improve Validation of Booleans

[RichardWarburton]

FIX only allows Y and N values for boolean fields. Currently we just check if the field value is Y. Ideally we want a way to validate boolean fields.

One option would be to throw an exception on the getter. Normally in Artio codecs we have methods for checking whether fields are valid rather than just throwing. So we could have a field method boolean foo() and possibly supplementary methods char fooValue() and boolean isValidFoo().

No matter the API we probably need to store the char value for booleans, rather than just a boolean.