enables multihit start timing treatment.
If non-zero, output class is art::TTimeDiffernce, otherwise, it is
identical to input. In the latter case, both Get/SetTiming() must be
available in that class.

now you can use TTimingChargeCalibrationProcessor more generally.
It only requires input of inheritance from art::ITiming and/or
art::ICharge and availability of all of their (Set|Get)(Timing|Charge) Method.
(you needed inheritance from art::TTimingChargeData before.)

google test and some tests are added in src/test.
You can make and execute test program by "make test" in src_test.
For detail, see source files.

if fNPlane < CRITICAL_NPLANE, 2-parameter tracking will be used.
a and b are assumed to be 0.
if fNPlane >= CRITICAL_NPLANE, 4-parameter tracking will be used.

for convenience of making new processor, mainly in Init().
For usage, see some processors written by Kawase.