Support for Pico offset limits

[azonenberg]

The PicoScope 6000 series (and likely others) loses offset range as you reduce the voltage range. For example, 50 ohm inputs at range 6 and below support a max of +/- 1.25V offset.

[azonenberg]

@preamp-org can you have a look at doing this?

[preamp-org]

Will do.