Update lcls2 interface for newest daq (seq_ctl)

[ZLLentz]

Expected Behavior

• support seq_ctl
• send readout_count, group_mask, step_group on configure
• closely scrutinize changes to BlueskyScan.py in the lcls2 repo
  • ensure we're using the detname to determine the step_group

Current Behavior

No support for the newer interface requirements or options

Context

lcls2 interface won't quite work correctly with the latest daq

[ZLLentz]

My notes from a meeting we just had:

- What changes?
  - Added seq_ctl arg as an optional way to tell when a step is done in a scan
    - Give it the PV to check e.g. for the burst sequence
    - Give it the value you want it to assert on each step
    - e.g. DAQ:NEH:XPM:0:SeqReset 4 = wait for SeqReset bitmask to be set to 4 = 0b100
    - Check out the XPM scanning page (Sequence Programming)
      - May be a shorter summary added to confluence later
    - What else would we set this to? Usually we look at StepDone internally
      - If using the accelerator to burst, it will be a different PV
      - SeqDone for when you're using an XPM sequence
    - Sent on configure
  - Some info that used to be sent on enable is now sent on configure
    - readout_count
    - group_mask
    - step_group
  - When scanning: default assume we're counting on the platform readout group
    - now an option: we can count on a slower readout group
    - set detname other than scan
  - Questions:
    - I guess we need to modify the interface to let you change the detname? It is hardcoded to be scan on master branch, I think you used to be able to change it.
      - Nah, this is ok I read the code wrong
    - If we set readout_count in configure, does that mean we always need to reconfigure if we change the readout count? (I realize that changing the readout count per step is not something that is commonly or ever done)
      - We can still change these things on enable as long as they were in the configure

Link to the repo:
https://github.com/slac-lcls/lcls2/tree/master/psdaq/psdaq/control