Bug: Add compatibility for latest version of sssom

[joeflack4]

Overview

I've fixed the sssom dependency to 0.3.28 because of some error that happens in newer versions. This will cause conflicts in some users' environments.

See this error message that appears if doing a fresh install or upgrading sssom to the latest version:

ERROR: pip's dependency resolver does not currently take into account all the packages that are installed. This behaviour is the source of the following dependency conflicts.
owl-on-fhir 1.1.5 requires sssom==0.3.28, but you have sssom 0.4.9 which is incompatible.