Add flag hdc_fix_propcorr so when set to 1  the problem in HTRACKING in which if
    when a hit is used in multiple spacepoints its drift distance is corrected for
    propagation time multiple times.
Merge pull request #1 from MarkKJones/mkj

Add flag hdc_fix_propcorr so when set to 1  the problem in HTRACKING in ...