clean build: dsl/model -- fix javac and javadoc warnings

This is the beginning of an effort to cleanup the OmeroJava build now that OmeroPy has been flake8'd and OmeroCpp has had all compiler warnings removed (gh-3145). The focus is on the numerous warnings produced by the generated code. All tests should remain green and there should be no functional changes anywhere. The only noticeable difference should be that running the ant build should be significantly quieter.