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

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• 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/doc/tutorials/Data-Insertion.rst

@@ -224,14 +224,14 @@ list-valued attribute in Python, as the following example illustrates.
 
 
 .. note::
-  Properties of Entities, that shall be updated, need to have IDs. Let's look at an
+  Properties of Entities that shall be updated need to have IDs. Let's look at an
   example:
 
 .. code:: python
 
    experiment = db.Record(id=1111).retrieve()
    experiment.add_property(name='date', value="2020-01-01")
-   retrieved.update() # Fails! The date Property needs to have an ID.
+   retrieved.update()  # Fails! The 'date' Property needs to have an ID.
 
 The easiest way to get around this is to use the corresponding entity getter:
 
@@ -239,10 +239,10 @@ The easiest way to get around this is to use the corresponding entity getter:
 
    experiment = db.Record(id=1111).retrieve()
    experiment.add_property(db.get_entity_by_name('date'), value="2020-01-01")
-   retrieved.update() # Works!
+   retrieved.update()  # Works!
 
-There are also the functions ``get_entity_by_path`` and ``get_entity_by_id``. You can easily use
-cached versions of those functions (see :doc:`Entity Getters<Entity-Getters>`).
+There also are the functions ``get_entity_by_path`` and ``get_entity_by_id``. You can easily use
+cached versions of those functions (see :doc:`caching options<caching>`).
 
 
 File Update