[IMP-dev] Suggested IMP cleanups

I would like to propose getting rid of the trajectory log in the model as we can do the same sort of thing using State classes. Since these aren't part of the Model, they would be much more flexible and customizable. I currently have such a state for writing chimera BILD files that I can submit at some point.

And I would propose removing the ModelData OptimizedIndexIterator (or whatever it is called). The API is funny and it doesn't provide any functionality which can't be achieved other ways using more or less the same number of lines of code.