Cannot use DAQ in scan with events=None, duration=None

[ZLLentz]

Expected Behavior

events=None, duration=None should be the defacto way to run the daq "until all other objects are done reading out", e.g. until the sequencer is done, etc., so you don't have to remember any magic numbers.

Current Behavior

This codepath is explicitly blocked at

pcdsdaq/pcdsdaq/daq.py

Lines 355 to 357 in eb30cca

	if all(cfg[key] is None for key in ('events', 'duration')):
	raise RuntimeError('Cannot start daq in scan step, did not '
	'configure events or duration.')

So to run forever, you need to pass events=0 or events=-1 in scan step
So this block doesn't even work, it's just annoying

Possible Solution

Think about this a bit and see if there is a way to stop someone from making a scan that barely works, or if these cases are ok and just remove the check here