Remove a bunch of math methods in SGVector.

These methods in SGVector (sin, cos, cosh, atan, abs, exp, log, etc) are barely used in Shogun; I only had to change two calls to `SGVector::log` and one call to `SGVector::exp`. In fact, IMHO, in C++ one should just write a loop through all the vector elements instead of having these convenience methods that are typical in e.g. Python, Matlab, etc. It should still be possible to resort to Eigen3 if these "vectorised-styled" calls are badly wanted for some reason.

In addition, it doesn't make much sense to keep them to provide access to them from interfaces. The typemaps are mapping the data in SGVectors to data structures in the target languages where these operations are normally already available (like `numpy.sin(a numpy array)`).
GP stuff all has hard dependence on eigen3. Why not use eigen3 here? It vectorises things and might be faster? Same for the other places