[Feature] Add UnitTests

[seb-esser]

Add UnitTest Framework using the unittest framework.

Initial tasks:

• initialize required infrastructure (add packages, update readme, ...)
• write tests for IfcGraphInterface to ensure that any kind of node and edge is correctly parsed in any graph provider available
• create tests to test the diff and and patch algorithms

Prepare everything under a dedicated feature branch and stage for merge upon completing this task.

[helenshz]

I skipped the inline entity test for now because of the following problem: In real IFC files, inline entities have no STEP ID, so id() returns 0. However, create_entity method always assigns a positive temporary ID, making it impossible to verify id() == 0 on the instance. Attempting to manually set id fails because it is read‑only.