[WIP] Fix LeCroy WaveSurfer 3k

[qberthet]

Attempt to fix the support WaveSurfer 3k serie of scope.

Initially reported in #996 (and then partially reported in #1026 and #1028).

Most annoying issue is fixed in the current state : averaging is not present per channel but under app.Acquisition.Horizontal.AverageSweeps. For now I have naively updated this path for the WS3k, but I suppose that this might cause issues if ngscopeclient expect an average setting per channel. Seems to work tho, but eager to implement a nicer fix once all the discrepencies with other LeCroy will be identified.

Probe detection seems fixed, it was caused by abscence of probe mux being reported in differently than in other LeCroy scope: no .ActiveProbe property (and a differentt property for probe name).

Still investingating:

• sample rate change does not work (event from WaveStudio)
• channel enable/disable
• intermintent crashes when changing memory depth
• and probably more :-)