ENH: new `page_length` parameter for execute_query

Summary

Related issue: https://gitlab.indiscale.com/caosdb/customers/geomar/management/-/issues/159

Focus

• Everything is designed to be used with list comprehension features. See usage examples in caosdb-pyinttest!65 (merged)
• The execute/execute_query functions will now return a Container, an integer or a generator for Containers depending on the parameters.

Test Environment

Tests in caosdb-pyinttest!65 (merged)

Check List for the Author

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
  • How do I use the software? Assume "stupid" users.
  • How do I develop or debug the software? Assume novice developers.
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.

src/caosdb/common/models.py

@@ -4391,7+4391,7 @@
             Whether to use the server-side query cache (equivalent to adding a
             "cache" flag) to the Query object. Defaults to True.
         page_length : int
             Whether to use paging. If page_length > 0 this method returns a
             generator (to be used in a for-loop or with list-comprehension).
             The generator yields containers with up to page_length entities.
             Otherwise, paging is disabled, as well as for count queries and
@@ -4421,7+4421,7 @@
         query_dict["query"] = str(self.q)
 
         has_paging = False
-        is_count_query = self.q.lower().startswith('count')
+        is_count_query = self.q.split()[0].lower() == "count" if len(self.q.split()) > 0 else False
 
         if not unique and not is_count_query and page_length is not None and page_length > 0:
             has_paging = True
@@ -4434,9 +4434,7 @@ class Query():
         self.cached = cresp.query.cached
         self.etag = cresp.query.etag
 
-        if is_count_query and len(cresp) == 0:
-            # this was a count query
-
+        if is_count_query:
             return self.results
 
         if raise_exception_on_error:
@@ -4480,8 +4478,8 @@ def execute_query(q, unique=False, raise_exception_on_error=True, cache=True,
         Whether an exception should be raised when there are errors in the
         resulting entities. Defaults to True.
     cache : bool
-        Whether to use the query cache (equivalent to adding a "cache" flag).
-        Defaults to True.
+        Whether to use the query server-side cache (equivalent to adding a
+        "cache" flag). Defaults to True.
     flags : dict of str
         Flags to be added to the request.
     page_length : int