Add a MODELRATE_CPS ( CPS ) variable to the model build.  Use this to calculate
dynamic limits for TIME0.

Change TIME0 measurement to always run on second cycle no matter the rate, and to
take rate into consideration for the calculation.

Fixes both fast (524K) and slow (33K) IOP models, specifically aimed at making LLO pemmid look good.

This code can be used when models run in userspace and without GPS hardware.
It removes the dependency on the special device file /dev/gpstime.
GPS time is obtained in userspace from the system clock.
Enable at compile time by defining USE_GPSCLOCK.
When building a model, add userspacegps=1 to the CDS parameters.