ENH: For set outputs ignore the order

[bsipocz]

It would be nice to have set comparison as opposed to adding +IGNORE_OUTPUT for lines like this one where the order in the output is not fixed:

305   >>> res = registry.search(ivoid="ivo://org.gavo.dc/flashheros/q/ssa")[0]
306   >>> res.access_modes()
Expected:
    {'ssa', 'tap#aux', 'datalink#links-1.0', 'web', 'soda#sync-1.0'}
Got:
    {'web', 'ssa', 'soda#sync-1.0', 'tap#aux', 'datalink#links-1.0'}
``

[pllim]

Maybe access_modes() should return a sorted list? It would be confusing to me as user if the order is non-deterministic.

[bsipocz]

yeah, I'll cross open a pyvo issue. Still, this maybe a useful feature for doctestplus.