[Numdiff bindings] std::vector of datas needs to be exposed in python

Currently, the bindings for numdiff/diff-action do not contain an exposure for std::vector. Thus, when I try to access DiffActionNumdiffData.data_x, which is a vector of datas, I get the following error:

In [3]: dnum.data_x
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-9ebbe2eabe85> in <module>()
----> 1 dnum.data_x

TypeError: No to_python (by-value) converter found for C++ type: std::vector<boost::shared_ptr<crocoddyl::DifferentialActionDataAbstract>, std::allocator<boost::shared_ptr<crocoddyl::DifferentialActionDataAbstract> > >