Refactor SimulationBuilder

[pbauman]

I like having the builder modules in SimulationBuilder because it keeps the Simulation API simple and makes it easy to extend We should just provide (const) accessors to each of the builder modules instead of redoing every call to the builder module. Nothing should be cached in the builder classes anyway (and if there is, we should fix it or have a damn good reason for it being there) so we can make everything const.