Implement a getter for the list of ordered steps

[S-Dafarra]

Right now, in orderSteps we generate a vector of ordered steps. This vector contains iterators to the StepList provided as input to the FeetInterpolator class. This allowed to avoid copies of Step objects, but if we need to share information on the list of ordered steps, passing iterators does not seem to be the best solution. This problem arose when we had the need of generating another set of references (see #3). In addition, the starting position of the feet is not included in this vector, which makes its semantic a bit unclear.
We should try to implement an efficient getter method to retrieve the information about the order of the steps, stating clearly whether the initial feet positions are included or not.

cc. @GiulioRomualdi

[S-Dafarra]

After #15 and #17, this is no longer necessary.