Abstract methods for difference and integrate state operators

I explored the option to create an abstract class that describes the state and its operators. I prototype quickly this solutions, however I found that the code is overcomplicated without any reason. For instance, it's need to define the state data (inside DynamicsData and CostData) using this abstract class. This also affect few places where is needed to write and read this information (mainly inside the DDP solver).

Despite this is a reasonable solution, I found it very complicate for users without gaining anything. Instead I decided to pursue the idea of having these operators inside the dynamic class; in the end the state definition it's also part of the dynamics definition.

These two abstract functions are called differenceState and integrateState. I'm not fully convince by this names but I have no better idea.

Please note that this solves the task #26 (closed).