Also, consistently use 'ch' for channel variables. This matches how we
consistently use sdi, devc, and so on all over the code-base.
This fixes parts of bug #259.
The name 'probe' (and thus 'probe group') is a relic from the times when
sigrok was mostly about logic analyzers. Nowadays we support a lot more
device types where 'probe' is not really a good term and 'channel' is
much better suited.
This fixes parts of bug #259.