gg: make ContinuousScaler work for any domain type
gg: make ContinuousScaler work for any domain type

Currently the methods of ContinuousScaler take float64s, but this
doesn't generalize to other continuous domains (e.g., time.Time).

Fix this by making its methods take interface{}. Document the meaning
and requirements of this value and the behavior of these methods.

aclements/go-gg,quentinmit/go-gg