Change issues file location in test

The issues moved from data.json to issues.json
rename Aspect to WeightedAggregate

Terminology confusion. I’m not sure this is the best name either, but
at least it’ll differentiate it from an Aspect of an Issue.
move Issue.motion_ids

Should always be dealing with Aspects
Remove Issue.aspect_for

Inline it, so we no longer need to be dealing directly with Issues, and
can move to working with Aspects.
variable renaming

renamed Aspect class, so rename the variable to go with it
Switch tests to Issue.aggregate_on
Deal with Aspects rather than Motions
JSON requirement → Aggregate

in wrong place
make_*_stances should make calls on Issue directly

not peer into WeightedAggregates.