Fix some clearly broken Tek5014 code in MixerOptimizer

Wasn't updated with the new Tek5014 driver.
--BRJ
Digital Attenuator read fixes

A second after connect, input buffer had "ERROR: Unknown command". This
change clears the input buffer before queries, so one gets back the
desired response, at least when used interactively. May need to find a
better fix for programmatic usage.

--BRJ
Allow cal_scale to work again with passed data.

--BRJ
Print instrument connecting messages in SingleShotFidelity

--BRJ
Merge branch 'develop' of github.com:BBN-Q/Qlab into develop