Software trigger routine for keysight 34465A and Keithley sourcemeter

[majoqubitT10]

It would be very useful to have software triggered readings into data buffer for the dmm and sourcemeters similar to how that is done for the SR830 as an alternative to the sync/timed readout already implemented

[WilliamHPNielsen]

Okay, I'll give it a look.

[WilliamHPNielsen]

The Keysight does not gain any speed upon being software triggered. See #729.

[WilliamHPNielsen]

The Keithley can be made twice as fast (assuming some mode of operation). See #747.