Excessive emptyElementParsePolicy warnings

If a schema has a need for the emptyElementParsePolicy property, but it's not defined, you end up with a lot of duplicate warnings like:

[warning] Schema Definition Warning: Property 'dfdl:emptyElementParsePolicy' is required but not defined, using tunable 'treatAsEmpty' by default.

This warning is helpful, but it is repeated for what seems like every element in the schema, which makes it not very user friendly.

Maybe we should only record the first instance of each WarnID, but add a warning that other instances were hidden. And add a tunable flag to enable recording of all instances if the user wants it all.

Additinoally or alternatively, we might want to think about not recording warnings at all for extensions. They are extensions after all. If someone's schema is DFDL v1.0 compliant, there's an argument that we shouldn't output warnings for things related to extensions.