[python/r] Adding/checking `dataset_type=soma` in SOMA Experiment metadata

[johnkerl]

As on #2884 :

• We used to write this in Python; we don't currently, which was an inadvertent mod
• We need to add a unit-test case to check that we don't drop it again
• For R we do write it currently: https://github.com/single-cell-data/TileDB-SOMA/blob/1.13.0/apis/r/R/SOMACollectionBase.R#L39-L45
• But for R:
  • ~We should do this for SOMAExperiment only, not all collections;~ We are doing this for SOMAExperiment only -- which is good
  • We should add a unit-test case (as we need to add in Python) to ensure we don't lose this

Note: this metadata used in the TileDB Cloud UI. We should write it regardless of storage-URI type, since users can ingest directly to Cloud, or, ingest to S3 and then register.

[mojaveazure]

As seen at https://github.com/single-cell-data/TileDB-SOMA/blob/1.13.0/apis/r/R/SOMACollectionBase.R#L41; this is only happening for SOMAExperiment objects in R, not all collections

[johnkerl]

LOL @mojaveazure I grepped and missed the if on the line above -- sorry for that! :)

Anyway I'll update the description