Allow to set `codeobj_class` for custom operations
Fix the interpretation of multiple stochastic variables in explicit state updaters. Closes #330
Allow explicit state updaters to have additional custom checks
Have the milstein stateupdater check that the noise is diagonal
Do not fail with a KeyError if an expression does not refer to all noise variables
Further fixes for the multiple stochastic variables situation
Make tests pass again, slightly refactor the `ExplicitStateUpdater` code (but `_stochastic_part` and `_non_stochastic_part` are still very redundant)
Add some basic tests for stochastic state updaters and multiple stochastic variables
Add a slightly more elaborate test for multiple noise variables where the random number generation is replaced by a constant value
Re-introduce a test fix, mark the more complex SDE tests as "long"