Added the action model for impulse forward dynamics + various improvements

This PR tackles issue #242 (closed). However, it improved the library in many terms, e.g.

• Refactored the way of sharing memory with cost sum. Now it can be used for both differential action and action models.
• Removed qcur and vcur in differential action models. These variables are not needed by using properly Eigen.
• Introduced the concept of autonomous systems (nu = 0), note that impulse dynamics are autonomous systems.
• Changed notation of variables in contact and impulse models. It's compatible with impulses and easier to understand.
• Added function to update forces, accelerations (or next velocity) and their derivatives for using later in cost functions. This is ongoing work.

These functions is a partial works that handles #96 (closed).