add helicity timeline

[baltzell]

closes #160

[c-dilks]

Looks like something is wrong with the output file creation:

• out_HELICITY_$runnum.hipo should be written to the corresponding run-specific subdirectories, along side of files like out_BAND_$runnum.hipo
• I don't see the helicity_efficiency.hipo file

I think both of these are the responsibility of bin/run-detectors-timelines.sh, but there may be more involved