Support coordinate systems

[szabo137]

Currently, only cartesian coordinates are supported by the Lorentz vector interface. However, in some situations, it is more convenient to initialise and use Lorentz vectors in other coordinate systems.

Suggested coordinate systems to be supported

Besides the cartesian coordinates, the following coordinate systems would be conceivable:

• spherical coordinates: Energy $E$, abs. value of the three momentum $\varrho$, cosine polar angle $\cos\theta$, azimuthal angle $\phi$
• transverse coordinates: (pseudo) rapidity $y/ \eta$, transverse energy $m_T$, transverse momentum $p_T$, azimuthal angle $\phi$
• light code coordinates: minus component $p^-$, plus component $p^+$, transversal components $p_x, p_y$.

Additionally, the implementing transformations between those coordinate systems are necessary.

Suggested implementation

Since it is convenient to not limit the four-momentum types to a common abstract super type, the coordinate systems above could be implemented as traits. This would be analogous to the current implementation of LorentzVectorLike which is essential the trait for cartesian coordinates.
For each coordinate system trait, a set of accessor functions could be implemented, which by themself are used in the Lorentz vector library, simliar to the current implementation.