Having consistent APIs

I believe we need to work on the API and its consistency. I saw few places where it's not intuitive or confusing. Here I want to point out an specific example.

DifferentialActionModelFloatingInContact needs during its construction process to pass the costModel. On the other hand, DifferentialActionModel (for ABA) has inside its body the cost model. Then, we need to use addCost function to deal with this.

Personally I'm with the first method. But both methods are good for method.

I want to use this issue as a remark that we need to work on the API (need for a milestone). I can deal with this task the next days. We might discuss and agree with the easy, and yet efficient, API that we should use.

@nmansard @rbudhira

PD: there are many other inconsistencies, including the filenames.