Helicity projection for unstable particles

[kilian]

Supplementing the current decay and polarization options (full correlations, diagonal projection, uniform decay), there is user request for projection on a specific (possibly correlated) helicity state(s).

This is a straightforward enhancement of the 'factorize_interaction' procedure, which we didn't fully exploit so far. Let's see if I can come up with a convenient user-level syntax.

[kilian]

Maybe this can be done on short timescale ...

[kilian]

A possible algorithm could rely on selecting polarized outgoing particles in an existing event sample. It turns out that there were two problems: all particles were printed as polarized if ?polarized_events was set, and selection expressions (e.g., selecting a polarization) were not respected when writing external event files. These issues are resolved in r6580.

The original issue is not addressed yet, unfortunately: we can't simply read in polarized samples and let them decay, the information in the event sample is insufficient.

[kilian]

Implemented in r6586. On the user level, the projection procedure is triggered, for a specific decay, by setting decay_helicity to a definite value, preferably as an option to unstable. I revised the decays section in the manual, accordingly.