EmbeddedErrorgen coefficients method missing return_basis support

[coreyostrove]

The EmbeddedErrorgen modelmember's coefficients method is supposed to support an option called return_basis which makes it so that a Basis object associated with the error generator coefficients is returned, but it seems this isn't implemented or is otherwise broken (don't recall exactly which as this was discovered in passing while developing stuff for PR #538). Not super high-priority at the moment, but should be tracked down and added eventually.