Fix timer in user IOP

The previous version did not handle overflow correctly, which resulted in large excursions in the cycle time.

Here is a plot of IOP cycle time in nanoseconds, without the fix. This is supposed to be 15259 ns for a 65536 Hz IOP. Big glitches are seen every second as the tv_nsec value overflows.

And here is how it looks with the change applied.

added 8 commits

• 1b78890d...f0e1f3fc - 5 commits from branch cds:master
• cd7cccec - Fix timer in user IOP
• 8c4e8611 - Increment gps sec before sending ADC/timing data to user models
• 82a78b4d - Spin instead of usleep to avoid any task switch

Compare with previous version

Updated to fix IPC errors seen with the userspace IOP and user models

mentioned in issue christopher.wipf/docker-cymac#8 (closed)

mentioned in issue christopher.wipf/docker-cymac#6 (closed)

merged

mentioned in commit 477d1d1b