Unbound functions for filtering particles

Functions
Particles &	Rivet::iselect (Particles &particles, const Cut &c)
	Filter a particle collection in-place to the subset that passes the supplied Cut.
Particles	Rivet::select (const Particles &particles, const Cut &c)
	Filter a particle collection in-place to the subset that passes the supplied Cut.
Particles	Rivet::select (const Particles &particles, const Cut &c, Particles &out)
	Filter a particle collection in-place to the subset that passes the supplied Cut.
Particles &	Rivet::idiscard (Particles &particles, const Cut &c)
	Filter a particle collection in-place to the subset that fails the supplied Cut.
Particles	Rivet::discard (const Particles &particles, const Cut &c)
	Filter a particle collection in-place to the subset that fails the supplied Cut.
Particles	Rivet::discard (const Particles &particles, const Cut &c, Particles &out)
	Filter a particle collection in-place to the subset that fails the supplied Cut.

Detailed Description