Add warning/error if extension cannot be loaded from namespace alone

[rly]

Suggestion from @bendichter while using the ndx-icephys-meta extension.

Some extensions cannot be loaded from the namespace alone because dynamic class generation doesn't work for a particular type or the class has custom ObjectMapper mappings that the extension does not know about. We should add a flag in the extension namespace that is True if the extension needs to be installed through pip install [extension] and import [extension] before reading the file. When reading a file with the flag and the extension is not already loaded, HDMF should raise a warning/error saying how to install the extension.

[rly]

Since we have substantially improved dynamic class generation, this should now be a rare occurrence. See also NeurodataWithoutBorders/pynwb#1818 . We could resolve that issue through a flag on the extension catalog entry or the namespace itself as proposed here. The catalog entry may be better because it encourages people to publish their extensions in the catalog if they have already published it on PyPI.