mutable gravity

The value of the gravity vector in world frame is currently hardcoded in wolf.h:

inline const Eigen::Vector3d gravity(void) {
    return Eigen::Vector3d(0,0,-9.81);
}

When switching between simulations and/or real datasets that use different values, this requires for the moment to painstakingly recompile core with the according value. One solution would be to put it as an attribute of one of the wolf objects and add a setGravity(Vector3d g) method (Problem being the most obvious candidate). However, this function is called in imu_tools.h for instance, which does not have access to the Problem.