Count references

[sbilge]

This PR adds the functionality of counting references transformation. It counts how many target objects are referenced from each source object given the relation name and adds the value to the corresponding datapack-content-properties. Since it builds on top of the "add content property" transformation, it assumes that the "path to target content(s)" is already present in its input SchemaPack and DataPack.

[coveralls]

Pull Request Test Coverage Report for Build 10594027772

Details

• 132 of 147 (89.8%) changed or added relevant lines in 14 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.7%) to 92.674%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/metldata/builtin_transformations/count_references/data_transform.py	18	20	90.0%
src/metldata/builtin_transformations/count_references/model_transform.py	15	19	78.95%
src/metldata/builtin_transformations/count_references/assumptions.py	48	57	84.21%

Totals
Change from base Build 9127000732:	-0.7%
Covered Lines:	759
Relevant Lines:	819

---

💛 - Coveralls

[mephenor]

Source vs. target needs to be changed to active vs. passive and both directions handled, as discussed.
In addition, there is some documentation that needs a bit more work put into.

[mephenor]

Should be good now apart from one tiny thing.
Up to your discretion if you want to change that