Integrate dynamic cast in aligator

[edantec]

Currently 3 different dynamic casts are needed to access a residual from the vector of stages (for example, to set a reference):

• Casting the CostAbstract held by a stage to a CostStack object
• Casting the component to a composite cost (usually QuadraticResidualCost)
• Casting the underlying residual

We could define template functions (eg: getComponent and getResidual) to access the residual more easily.

[ManifoldFR]

PR #199 will take care of your second point @edantec. PR #198 took care of your third point.