Centre triggers for PandA Flyer

[huw-dls]

Expand the PandA Flyer to use two rows for the immediate triggers (instead of one). The triggers can then be centred with the livetime always being surrounded by 1/2 deadtime. This would also align the odd and even rows of snake scans.

[RJCD-Diamond]

This could be a breaking change for anyone using a panda , and would probably require changes on the pandablocks-ioc too. Could you just set time2 to 0 and just treat each seqrow as 1 trigger/1 time row.

[coretl]

This only affects those who use ScanSpecSeqTableTriggerLogic, and requires no changes to the IOC, it is the addition of a single line here:

ophyd-async/src/ophyd_async/fastcs/panda/_trigger.py

Lines 144 to 151 in 182780b

	# Time based Triggers
	rows += SeqTable.row(
	repeats=end - start,
	trigger=SeqTrigger.IMMEDIATE,
	time1=int((slice_duration[0] - value.deadtime) * 10**6),
	time2=int(value.deadtime * 10**6),
	outa1=True,
	outa2=False,

However you make a good point that for scans that don't move a motor, should the triggers be centred on the scan frame? We should think about that...