Make tekscope a triggerable device

[chrisjbillington]

The TekScope class is typically used by wiring a DO to it as a trigger input and commanding an edge on that DO when you want to acquire.

The class could simplify this by being itself a TriggerableDevice, such that you would call TekScope.trigger(t) rather than keeping track yourself of which DO was the trigger.

Should be possible to make this backward compatible by allowing the DO arguments to be omitted, in this case .trigger() would raise an exception.