Automatic units rescaling in live plot

[AdriaanRol]

I just learned that PyQtGraph (which we use for live plotting) supports auto rescaling of unit labels if you specify a unit in the label! (see microsoft/Qcodes#495).

I've tested this quickly (see screenshots) and it automatically puts prefixes such as Mega, micro and nano in front of the labels. I think this is an absolutely must have for our live plotting. I'll probably do this somewhere in the next few weeks. (hopefully in a nice way and otherwise in a hacky way).