Defining an abstract class for action models and datas

The main target of this PR is to create an abstraction class for action models and datas as pointed out here #79 (closed). The tasks achieved were:

1. Creation of abstract classes for action models and datas.
2. Used this class to implement unicycles, LQR and NumDiff classes (both model and data)
3. Added g and L variables in differential action and action data classes. This is convenient for saving time in some cases.
4. Used standard naming for AMLQR formulation.
5. Removed setUpRandom from AMLQR. With this, we preserve the same structure of DAMLQR.
6. Added recalc option for AMLQR.calcDiff.
7. Removed UnicycleData classes since we don't need special implementatio (with the abstract it's enough).