Introduce InstrumentModule as a superclass of InstrumentChannel

[jenshnielsen]

And clarify (via typing) that submodule in an instrument must be either InstrumentModules or ChannelLists

Open questions:

• Should we retype ChannelList and friends to allow InstrumentModule as a type. Not right now. We may add more channel specific functionality to InstrumentChannel down the line
• Update docs

[codecov]

Codecov Report

Merging #3840 (c7e98d3) into master (11c7614) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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##           master    #3840   +/-   ##
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  Coverage   65.57%   65.57%           
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  Files         227      227           
  Lines       30641    30643    +2     
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+ Hits        20093    20095    +2     
  Misses      10548    10548           

[astafan8]

Should we retype ChannelList and friends to allow InstrumentModule as a type

hmm. so that channellist may contain instrument modules, not just instrument channels? if so, then i'd probably wait with that since i can't think of an instrument were i'd want to have a channel list of non-channels, e.g. of things/modules that don't have a port on the panel of the instrument.

Update docs

yes! :)

also need to add a newsfragment :)

[jenshnielsen]

@astafan8 Thanks docs are updated and newsfragment added