Create templated versions of EigenHelpersConversions helpers

[dmcconachie]

For example, right now we would need 4 different functions for the following:

inline geometry_msgs::Pose EigenIsometry3fToGeometryPose(const Eigen::Isometry3f& transform);
inline geometry_msgs::Pose EigenIsometry3dToGeometryPose(const Eigen::Isometry3d& transform);
inline geometry_msgs::Pose EigenAffine3fToGeometryPose(const Eigen::Affine3f& transform);
inline geometry_msgs::Pose EigenAffine3dToGeometryPose(const Eigen::Affine3d& transform);

There are many such functions that would need to be duplicated. I think templates is by far the better way to go for this.

[dmcconachie]

Instead of replacing the existing interface, we could instead leave the existing interface alone for backwards compatibility, and have the existing functions call the templated versions. Not sure if this sort of backwards compatability is valuable.