Fix list returned by statistics.get_stratified_stats_types().

Had previously changed the caller of this functions to remove ** parameter
magic, but this means that the object returned needs to be a list of tuples,
not a list of maps.
GROUPED_THRESHOLD parameter missing for 'analyse_runs' command.

Which meant its default value was not available. Also need to fix fact that
although validation was being run plain _PiquantOptions, the option map was not
being updated with the validated values (i.e. the value for GROUPED_THRESHOLD
would remain as a string, not converted to an int).
More finely-grained params for piquant.py plotting functions.