Move the collection backend from tripper to DLite

[jesper-friis]

Tripper and DLite are independent packages (meaning that pip install tripper does not install DLite and pip install dlite-python does not install tripper).

However, pip install tripper[testing] do depend on DLite (since the collection backend uses DLite), likewise does pip install dlite-python[mappings] depend on tripper. Hence, there is a very weak (or maybe non-existing, depending how you see it) circular dependency between tripper and DLite.

Question: Is this really an issue?

If the answer is yes, it can be resolved by moving the collection backend from tripper to DLite. In that case, use entry points to register collection as an external tripper backend. See https://github.com/EMMC-ASBL/tripper/blob/master/docs/backend_discovery.md#installed-backend-package.

Personally, I don't see this as a real issue. The problem with circular dependencies is that it can lead to tight coupling between modules, which would restrict code reuse and make long-term code maintenance more challenging. But in this case the circular dependency is very weak (or maybe non-existing), since it is only present when requiring extra dependencies.