Add missing exported classes to somacore

[johnkerl]

We need to export Measurement and Experiment in addition to Collection. While it is true the first two are specializations of the third, they are not all interchangeable. Specifically, we use soma_type: Final = "SOMAMeasurement" and soma_type: Final = "SOMAExperiment" as inherited in their analogous TileDB-SOMA classes. If TileDB-SOMA's Measurement inherits from somacore.Collection then its soma_type will be wrong; which is of course unacceptable.

Context: https://github.com/single-cell-data/TileDB-SOMA/issues/669

[johnkerl]

No longer needed on https://github.com/single-cell-data/TileDB-SOMA/pull/678

• I had issues with https://github.com/single-cell-data/TileDB-SOMA/pull/678 and 0.0.0a2 of somacore
• Then I went to main of somacore and made these mods
• But there was a lot of stuff between 0.0.0a2 and this week's main of somacore, more than the little bit I added between main of somacore and this PR

With @thetorpedodog 's able help we were able to get https://github.com/single-cell-data/TileDB-SOMA/pull/678 working with 0.0.0a2 of somacore.