Add 'has' filter to CaseCollection

[rwiker]

Issue

#325

Short description

1. The CaseCollection constructor and the method CaseCollection.filter() have a new optional parameter called has. The filter() method generally returns a new CaseCollection instance; if has is specified, it is passed on to the constructor, which stores it in the member slot _has.
2. All searches on a DocumentCollection involve the _next_batch()/_next_batch_async() methods. These methods are overridden by CaseCollection.
3. _next_batch()/_next_batch_async() for CaseCollection checks if the member slot _has is set. If it is, we
   • Run a query to get all the values for fmu.case.uuid for the existing filters for the CaseCollection instance.
   • Create a new query that combines the value of _has with the set of values for fmu.case.uuid. Note: this new query applies to all object types, and not just case objects.
   • Retrieve all values for fmu.case.uuid for this new query.
   • Change the current filter for the CaseCollection instance to something like {"ids": {"values": <uuids>}}
   • Set the _hasmember attribute to None.
4. After checking/handling the _has attribute, _next_batch()/_next_batch_async() passes control to the parent class implementation.

Pre-review checklist

• Read through all changes. No redundant print() statements, commented-out code, or other remnants from the development. 👀
• [✅] Make sure tests pass after every commit. ✅
• [✅] New/refactored code is following same conventions as the rest of the code base. 🧬
• [✅] New/refactored code is tested. ⚙
• [✅] Documentation has been updated 🧾

Pre-merge checklist

• Commit history is consistent and clean. 👌

[equinor-ruaj]

lgtm