[Halld-offline] new ffread card for HDGeant

[Simon Taylor]

Hi, everyone.

Many of you have probably noticed that since I introduced the "cluster" 
code in the hitFDC.c and hitCDC.c routines in order to simulate the 
behavior of clusters of electrons drifting in from various places along 
a particle's trajectory within a straw or FDC cell, hdgeant has slowed 
down considerably and the output files have ballooned.  To remedy this 
situation I have introduced a new FFREAD card called DRIFTCLUSTERS with 
the default value set to 0 (off) and made the relevant changes to the 
hit routines.   The cluster code is still available (DRIFTCLUSTERS=1) 
but is not recommended for use for the reasons stated. 

Simon