Warn when creating an extension that is not supported by `get_class`

[tjd2002]

(per discussion with @bendichter)

For extension creators, it would be helpful to detect when a created extension is not supported by get_class, and warn that custom python classes need to be created to work with the extension.

Currently, MultiContainerInterfaces are not supported, and Ben came up with a couple more pathological cases that would not be supported (e.g., IIRC, a spec group that is not an neurodata_type, but has the same name as a neurodata_type).

This warning could be issued:

1. When the .extensions.yaml file is created (NWBNameSpaceBuilder.export)
2. when a file is read with load_namespaces=TRUE
3. ? some other time