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.
The maximum sample size that can be set on a device is now published
by sr_config_list(SR_CONF_LIMIT_SAMPLES). This returns a tuple of
uint64_t representing minimum and maximum number of samples.