[Halld-cal] mcsmear TDC digihits

[beattite]

Hello.

I just submitted a pull request to apply the recent changes to mcsmear 
and sim-recon that include TDC digihits, and which fix the problem we 
had of HDGEANT outputting negative times (which are being filled into 
unsigned integers in the digihit containers).  Now, the times are offset 
before conversion to digihits and restored in the hit factories through 
the use of the BCAL/base_time_offset ccdb constants, variation=mc, which 
I recently set to -100 (fairly arbitrarily).  After the conversion, if a 
hit time is still negative, it is discarded.