Support for the PrawnBlaster pseudoclock

[philipstarkey]

This PR provides support for the new PrawnBlaster pseudoclock (firmware release in labscript-suite/PrawnBlaster)

PrawnBlaster Specs:

• Configurable as 1, 2, 3, or 4 truly independent pseudoclocks. Each pseudoclock has it's own independent instruction set - there need be no synchronisation between pseudoclock instructions. It is equivalent to having up to 4 PineBlaster boards on a single chip!
• 30,000 instructions (each with up to 2^32 repetitions) if configured as a single pseudoclock. Instructions are spread evenly amongst pseudoclocks if configured between 2 and 4 pseudoclocks (15,000, 10,000, and 7,500 respectively).
• Minimum pulse half-period 50ns (if internal clock running at 100MHz).
• Maximum pulse half-period ~42.9 seconds (if internal clock running at 100MHz).
• Half-period resolution (between the minimum and maximum pulse half-period) of 10ns (if internal clock running at 100MHz).
• Support for initial hardware trigger (can be used as a secondary pseudoclock in the labscript-suite).
• Support for up to 100 retriggers mid-execution (labscript-suite waits) per independent pseudoclock.
• Support for timeouts on those waits (with maximum length 42.9 seconds).
• Ability to internally monitor the length of those waits and report them over the serial connection at the end of the instruction execution.
• Can be referenced to an external frequency source.
• Internal clock frequency can be increased to 133 MHz (scale above timing specs accordingly).

Support for all of these features should be included in the PR.

[philipstarkey]

@chrisjbillington Since this is new code, I will probably merge it tomorrow unless you had any objections. That way #70 can be rebased and updated with docs for the PrawnBlaster.

[chrisjbillington]

No objections, happy to rely on your judgement!