tweaked trigger objects

[yiiyama]

• Slightly reduced memory footprint of trigger objects by only creating object pointer vectors for registered filters
• More importantly, added "ignoreMissing" flag. If true and an unknown filter is passed to filterObjects(), can return an empty vector.

[yiiyama]

Is tested in Monophoton analysis framework and working.

[mitsidekick]

Tests passed! Plots here:

• http://t3serv001.mit.edu/~jenkins/relval/?d=PandaPhysics/PandaTree/PR-130/1

[dabercro]

I meant to add a comment, but I can confirm that this doesn't change behavior of the MET triggers used in Hbb 0-lepton