more dynamic register assignment

[meeg]

Currently, qick_asm assigns the DAC pulse parameters to specific registers; the pulse setup writes to those registers and then the set commands read from those registers. These assignments are hard-coded, and there also isn't anything special done to make sure that nobody tries to use these registers for something else. Similarly, the loop counters go to specific registers.

If you're writing a program that needs another register for something, you need to know about these existing register assignments. It would be nice if you could instead request an available register (perhaps on the register page corresponding to the DAC you're working with), and qick_asm would pick a register number that your program isn't using.

From discussion with @circuitqed.