Improve `ProbabilisticModel` base class using abstract base class

The `ProbabilisticModel` base class (below) defines the interface that custom model classes must implement to work in `deepsensor` (such as `.mean`, `.stddev`). This should be updated to be an abstract base class using `collections.abc`: https://docs.python.org/3/library/collections.abc.html

https://github.com/tom-andersson/deepsensor/blob/e349aaa48f6ed673c721b13b64b9a7b422425876/deepsensor/model/model.py#L95-L108

cc @patel-zeel

Start with deepsensor/model/model.py at the linked ProbabilisticModel definition and review the collections.abc documentation. Update the base class so its required interface, including methods such as .mean and .stddev, is abstract, then verify that custom model classes can still implement the interface.

Written by the indexing model from the issue text.