[Halld-offline] updates to kalman filter code

[Simon Taylor]

Hi.

As promised in a couple of our meetings last week, I have just committed 
several changes to the Kalman filter code to deal with low momentum
tracks stopping somewhere in the tracking volume and added some code to 
try to recover "broken" tracks (by which I mean tracks for which the 
pruning removed too many hits or the code found some sort of kink in the 
track).    I have implemented a command-line
parameter -PKALMAN:RECOVER_BROKEN_TRACKS to turn on/off the latter 
feature.   Currently the default is set to "off" so that I (and others 
who are interested) can study the effects of this feature.

Simon