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.

warning: no treatment for multihit start timing

linkdef_user.h and makefile