[Halld-offline] recent changes to tracking code

[Simon Taylor]

Hello.

For those of you who missed my presentation at the offline meeting 
today, I would like to announce some recent changes to the tracking code 
that I have committed to the trunk:
1) I have added two command-line parameters to enable using the hits 
from the track candidate (as opposed to reswimming and using the hit 
selector algorithm to regather hits) as input to the wire-based fitting 
and to use the hits used in the wire-based fit as input to the 
time-based fitting.  The previous version of the code required 
reswimming and reselecting hits at both stages of the fitting.   By 
default these parameters are set to false (i.e., the hit selector 
algorithm will be used).  To turn on the usage of the candidate hits in 
wire-based fitting, set -PTRKFIT:USE_HITS_FROM_CANDIDATE=1.
To turn on usage of the hits used wire-based fitting as the list of hits 
to use in time-based fitting, set -PTRKFIT:USE_HITS_FROM_WIREBASED_FIT=1.
2) In the Kalman filter code for central tracks (with cdc hits only) I 
have restricted the reference trajectory to extend just beyond the 
outermost hit by a radial distance of 1 cm  (as opposed to swimming all 
the way out the BCAL).  The hope is that  this will help with low 
momentum protons stopping in the CDC.
3)  I have added parameters to control pruning of hits within the Kalman 
filter.     The parameters are based on the number of sigmas a hit is 
away from the projected position from the previous information in the 
fit.   These can be changed on the command line by setting 
-PKALMAN:NUM_FDC_SIGMA_CUT=xx and -PKALMAN:NUM_CDC_SIGMA_CUT=yy.     The 
default values are 20 and 10, respectively.
4) I have made the track candidate matching between the FDC and CDC more 
robust, but these changes should only affect a small percentage of the 
tracks.

Simon