splitBolus() is a model-only directive that rewrites bolus doses
aimed at cmt into parallel bolus doses to the target compartments.
Each target compartment receives the full original amount. The
source-compartment bolus is replaced, not retained.
This rewrite applies both to event tables translated by etTrans()
and to future doses scheduled during solving with evid_().
The source compartment may also appear in the target list, but the
target compartments in ... must be unique.
