GaussianStateUpdate is not an isinstance of StateUpdate

[gawebb-dstl]

>>> isinstance(GaussianStateUpdate([1],[[1]], None), StateUpdate)
False

This isn't intuitive (to me at least). In the above code StateUpdate can quite easily be replaced by Update.

This isn't a big issue but I wonder if StateUpdate is required. It's only usage within the ‘main’ branch is for tests. However it may be used in other branches or repositories.