QEE: persist pions in same event as $\pi^0 \to e^+e^-\gamma$

CURRENT Implementation

Allows background template to be evaluated offline via including more selective persistence.

• a container with all the \pi^\pm in the same event is persisted in the (already prescaled) full stream HLT2 lines
• Adds sprucing lines that are further prescaled after the full stream HLT2 lines which persist these pions (and the photons which were already being persisted in the Full-stream lines)

BANDWIDTH IMPACT:

Since the Hlt2 rates are not changed, they are not listed here. The "old" bandwidth indicates the bandwidth of the current master. The "new" bandwidth is the bandwidth that saves all charged pions in the same event.

Stream	"Old" bandwidth [GB/s]	"New" bandwidth [GB/s]	Rate [kHz]
Hlt2 Full steam	0.34998	0.36366	no change
Sprucing Prompt	-	0.0032	0.0569
Sprucing Displaced	-	0.00556	0.0569

Historical

Allows background template to be evaluated offline via including more selective persistence.

• A container with a random pion per event [above a cut] to be selectively persisted at the Turbo stage.
• A container with all pions per events [above a cut] to be selectively persisted at the Full stage.
• New Sprucing lines of similar strategy to the Full lines to be added.

Rec!3758 discusses a functor that would allow for this 'random selection' of a pion. As the discussions there were ongoing, chose to descope the Random Functor and Turbo Line persistence that depended on it.

LATER: The random selection of one charged pion in the triggered sprucing lines is intended to be done in a new merge request.

FYI: @johndan @tfulghes

#672