Membership class should not need to care about 'set mode'

[ots22]

At the moment this is only used in matching_intersections, and the results can be expressed in an alternative way that is the same whichever mode is used.

The mode is also used by the various plotting classes and currently this is taken from a Membership object. Since this is then basically only used as a flag to configure the plots, it would be better obtained from elsewhere, or passed directly to the plotting classes. A PlotSession has a member '_set_mode' which could be used for this purpose.