The Boolean value true if the current simulation
time is equal to the simulation start time. Otherwise, returns false.
If this S-function resides in an enabled subsystem configured
to reset states, and if the enabled system is enabled at the start
time, then mdlInitializeConditions is called a
second time and the Boolean value of ssIsFirstInitCond is true.
Use inside mdlInitializeConditions to determine
if the callback is being invoked at the start of the simulation.