add reference frame as tag for positional vectors

A nice addition to this library would be to define different Reference Frames in the same coordinate system type, e.g. cartesian. See here rationale:

https://stackoverflow.com/questions/34670464/how-can-i-add-coordinate-system-reference-frame-information-to-my-data-to-avoi

Similar strategies are already implemented in the SymPy library (Python) and in CERN's ROOT (C++), but I haven't found it in Boost, and might be an interesting feature to have taking profit of the boost/geometry formalism. It could be backward compatible, in the sense that if one does not define it, then it uses a DefaultReferenceFrame as template default parameter.

http://docs.sympy.org/0.7.2/modules/physics/mechanics/vectors.html
http://project-mathlibs.web.cern.ch/project-mathlibs/sw/html/Vector.html