Use LogicalMarker channels in a sequence

[caryan]

The user should be able to generate square pulses from the marker channels at will, not just special cases like digitizerTrig.

[blakejohnson]

This is very likely caused by the fact that we use magic entry keys (namely, PatternUtil.TAZKey and PatternUtil.markerHighKey... allow the latter is only checked implicitly) to encode information on marker channels. We need to make the code more flexible.

[blakejohnson]

I'm interested to see if the recent refactoring has improved this situation here. We have moved heavily away from relying on the magic TAZKey.