Add option to create a scope signal based on an expression.

[Federerer]

Sometimes it would be nice to plot something other than voltages (current, power) or simply add some scaling or offset for presentation purposes. Currently there is no way to do this (or I haven't found one). I think something in similar to the output channel's expression field would be OK?
This is mostly a note to self. I would have added an appropriate label, but it seems that I can't. 🤔