Zero-crossing type annotations

[DeadParrot]

QSS efficiency could benefit by having event indicator annotations indicating their zero-crossing type. With this information QSS could avoid the expensive zero-crossing protocol steps for a crossing of the wrong type.

QSS currently has this tentative enum for the categories but it may need refinement:

    enum class Crossing {
     DnPN = -4, // Positive to negative
     DnPZ = -3, // Positive to zero
     Dn   = -2, // Positive to <= zero
     DnZN = -1, // Zero to negative
     Flat =  0, // Flat zero value
     UpZP = +1, // Zero to positive
     Up   = +2, // Negative to >= zero
     UpNZ = +3, // Negative to zero
     UpNP = +4  // Negative to positive
    };