ENH: add entity getters and cached functions

Summary

Added:

• Entity getters: get_entity_by_<name/id/path> to retrieve single Entities identified via some attribute
• Cached versions of entity getters and of execute_query (cached_query)

Focus

You might want to look at the documentation first (build it with make doc). lru_cache is used for caching. lru_cache does not have the ability to add items to the cache manually. Thus, a dumb cache was added to allow this (see code).

Test Environment

unit tests.

I did not add integration test due to two reasons:

1. the getter functions are defined via the query that is sent. We do not need to test the query function here.
2. the getter functions and the execute_query function are mocked such that everything can be tested as unit test. (Those functions are the only connection to the server that would require integration tests)

Check List for the Author

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• Reference related issues
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• Up-to-date JSON schema (or not necessary)
• Appropriate user and developer documentation (or not necessary)
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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
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src/doc/tutorials/caching.rst

@@ -51,6 +51,8 @@ have entities on hand from previous queries that you want to add.
    # If you now use IDs that were in items, they are taken from the cache.
    e1 = cached_get_entity_by(eid=10001)
 
-For the cached entity getter functions (``cached_get_entity_by``) you need to set ``unique=True``.
+When filling the cache with Entity objects for ``cached_get_entity_by``, you need to set
+``unique=True``, whereas the cache for ``cached_query`` should be filled with Container object and
+``unique=False``.