Add ability to specify the group refiner(s) used in a canonical search

[wilfwilson]

Currently, with the standard GAP interface, you can specify refiners for the thing that you want to canonise, but Vole does its own thing with the first argument, i.e. the group G that you are canonising in.

It would be nice if you could tell Vole which refiners it should use for the group (does that even make sense?).