SL: Remove both topo and prescale from background lines

I was asked at the SLB meeting today to remove the prescale from the background sprucing lines: this will increase bandwidth. To alleviate this I was also asked to remove the background sprucing lines from triggering on the topo.

Overall this does more harm than good in terms of bandwidth:

Removing the topo changes our bandwidth by 0.94x compared to master. I believe this makes sense, the topo is supposed to identify signal, these lines are for backgrounds

Undoing the prescales on top of this changes our bandwidth by 1.34x compared to master Therefore we are proceeding instead setting the prescales of background lines to 0.5, including those which were previously not prescaled This changes our bandwidth to 1.07x compared to master

tagging: @groberts @amathad @gciezare @dovombru

SLB folk, do you have a suggestion on what could be done here?

DPA folk, this is work in progress, please don't panic at the bandwidth changes yet. I don't want to merge this both parties are happy.