Change to async SUMO calls

[anders-kiaer]

In progress. Currently blocked by equinor/fmu-sumo#224. Waiting on input on equinor/fmu-sumo#228.

Suggested pattern: Using a "factory method" (@classmethod) to initialize classes, and a parent-child-class-relationship which enables us to remove duplicated __init__ and from_case_uuid methods on access classes.

Other approaches tried:

• Using __aenter__ and async context managers (since async code can not run in __init__), however this is more complex and would require multiple levels of idendation in cases different data type access classes are required in an endpoint.
• Using @staticmethod. There is almost no difference to this approach compared with @classmethod. The benefit with the latter is that it is trivial, and the common Python pattern, to make factory methods on parent classes which can be used from child classes. With parent-child class relationship we can completely remove __init__ and the factory method on the child classes.
• We have CaseInspector and IterationInspector, in addition to the different access classes.
  • There exists endpoints which uses instance of CaseInspector, IterationInspector and an access class. This results in three different async calls to sumo to verity three times the case uuid is unique. To easily avoid this, IterationInspector is renamed to SumoEnsemble (which is also the mentioned parent class for the other access classes). The CaseInspector is renamed to SumoCase and made the parent class to SumoEnsemble again, but this inheritence level is not as important I think so could be removed if we prefer.

Closes #240.