Add more functions and control on the math plots

[msuciu]

I think it will be a good idea to add the Abs() function, reedit the math plot function, display it on the plot window as a label and maybe add more than 4 math plots.
Another think which could be useful is to perform some math operations on the captured signal when the oscilloscope is not running.

[damercer]

I would like to add a suggestion to be able to change / set the units on the math function. Such as change mV to mA when you divide the voltage from a channel by a resistance to get the current.

Doug