Improve notation of nested helicity angles

[redeboer]

Remove plus signs from helicity angle symbols

Helicity angle symbols use a subscript with a + sign, whereas invariant masses leave out the + sign (see here). The reason is that the meaning of these indices is slightly different: for invariant masses, they indicate addition of the four-fomenta for these final state IDs, and for helicity angles, they indicate reference frames. See get_helicity_angle_label() and get_invariant_mass_label().

The plusses however clog the notation of final expressions, see e.g. here:

This will notation will become even more bothersome when implementing #6. So the proposal is to remove the plus sign from this line:

Write rest frame as superscript

Currently, all the indices that come after the first comma in a helicity angle subscript indicate the reference frame from which the angle is computed. These can be written as a superscript, like so:

This notation is also used in some other papers, like Marangotto Eq. (21) (the superscript being the reference frame of the resonance). In addition, it's unlikely that one has to e.g. square an angle—the angles only appear as arguments to Wigner-D functions.

Some problems:

• Harder to implement and to get the LaTeX right, because of recursive_label().
• Names of these symbols become slightly more bothersome, even if it renders more nicely as LaTeX, e.g.:
  "phi_4,34,034" → "phi_4^{34,034}"
"phi_34,234" → "phi_{34}^{234}"
  In most cases, for a SymPy Symbol, something like this would still be valid LaTeX though: "phi_4^34,034" (this breaks once taking a square root of this Symbol)