changes to improve multiboard sync

[meeg]

Various changes to simplify/improve the use of multi-board synchronization.

• add a step_rounds argument to acquire() etc., to allow running one round at a time - needed for correct sequencing of setup steps
• add a get_rounds() method to grab the round-by-round results of a program
• tweak the standard tProc v2 firmware projects so the tProc core and timing clocks are in integer-multiple relation - needed for jitter-free synchronization
• fix a bug in tProc v2 r23 which led to bad DMA transfers if the core and AXI clocks were not related - needed for the previous item

Note that soft_avgs has been renamed everywhere to rounds in tProc v2, for consistency. This might break some user code. tProc v1 code can continue to use either as a config dictionary key.

[ghost]

@meeg is there any reason why you've removed the external proc_stop connection to the PMOD?

[meeg]

It's not used normally - typical programs have a finite duration and you want the whole thing to run, the only thing you want under external control is when the program starts. If you have some reason you want to force the program to stop, you will probably be doing that through software.

I originally wired both the start and stop ports because the tProc had both, but I heard from someone who was playing with external start; they saw spurious stops which they thought were coming from noise on the stop pin (which they had left floating). So, better not to wire it.