dummy import/export for GenotypeActivityBlock. PMT #96793

The GenoTypeActivityBlock doesn't actually have any state; it's just a placeholder for a template file with JS. But without any kind of import/export json/dict functions, pagetree ignores it on a dump + import.

Just needs a couple dummy methods added to make sure that it gets included.
Merge pull request #1 from ccnmtl/genotypeactivityblock-dummy-export

dummy import/export for GenotypeActivityBlock. PMT #96793