Libpitt/606 grabbing activity props

[libpitt]

Change log:

1. Set up a global property _schema_properties which is a dict with entity property names as keys so as to facilitate constant time access. Since we have various entities, it would be inefficient to loop every entity, then loop every property and match in order to group.
2. Simplify logic in trigger get_dataset_organ_and_source_info and speed up response

[libpitt]

So yesterday when I was testing the POST /descendants endpoint, in the results were both a Publication and a Dataset. In the filter I had requested title. But since title is a property of both Publication and Dataset, the previous code had a key collision. In Dataset, it’s a trigger, in Publication a neo4j property. In order to avoid loss, would have to catch the settings of each property per Entity type. This is right away going to be O n^x time unless we use a dictionary where we can get details about a property via a key, which is constant time. But we don’t want to be creating that dictionary every time we do a POST filter_properties type call. Hence the global variable _schema_properties and that method group_schema_properties_by_name that runs on startup.