move code that changes lognormal and uniform to Eidos to `slim_engine.py`

[petrelharp]

Currently the python code implements these as "type='s'" (eidos script); I think it'd be more consistent to do this sort of post-hoc processing in slim_engine.py, since we should separate out simulator-dependent details (e.g., "eidos script") from the high-level description (e.g., "lognormal").