Do some cleanup (e.g. delete unused imports)
Add a convenience function `add_clock_variables` to `Variables` that add the correct `Variable` objects for `t` and `dt`.
Do not notify the device about the creation of `Constant`, `AttributeVariable`, etc. It only needs to know about `ArrayVariable` and `DynamicArrayVariable`.
Start a different handling of the "implicit namespace" (now takes all frames *starting* from the calling frame into account). There's one remaining issue with warnings in `PoissonGroup`.