Update USDM report to differentiate between entity definitions and entity references

The Entity Details section of the USDM report (in both JSON and Excel formats) contains a list of "entities" with a count of occurrences. The numbers listed are based on the number of records found in each of the class-based datasets that are created when the USDM data service converts the input USDM JSON data to tabular format. However, the class-based datasets contain both:
- A record representing the definition of each instance of the class (`rel_type = "definition"`), and
- A record representing any reference (by id) to the instance (`rel_type = "reference"`).

It's not clear from the report that the Entity Details numbers include both definitions and references, so it would be better if the report were updated to make this distinction clear.