Retrieve trait information

Issue:
https://github.com/genenetwork/gn-gemtext-threads/blob/main/topics/gn1-migration-to-gn2/clustering.gmi

* gn3/db/traits.py: add functions to retrieve traits information
* tests/unit/db/test_traits.py: add tests for new function

  Add functions to retrieve traits information as is done in genenetwork1
  https://github.com/genenetwork/genenetwork1/blob/master/web/webqtl/base/webqtlTrait.py#L397-L456

  At this point, the data retrieval functions are probably incomplete, as
  there is more of the `retrieveInfo` function in GN1 that has not been
  considered as of this commit.

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