ofReadOnlyParameter operators should be public

Error [here](https://github.com/openframeworks/openFrameworks/blob/develop/examples/gui/parameterEdgeCasesExample/src/testApp.cpp#L30), because [*= operator](https://github.com/openframeworks/openFrameworks/commit/03ae024b3ced03fbca2d7212c3b009c9e9ac664a#L2R125) is protected, and unaccessible.

This fix it, by make operators public. Please **double** check if that follow that you want about `ofParameter`, @arturoc.