add method to transform coordinates by a RateMap

[petrelharp]

For instance, if we'd like to transform coordinates to be in genetic map units; as discussed with @lkirk @apragsdale @jeromekelleher .

[nspope]

couldn't transforming to map units be done by RateMap.get_cumulative_mass?

[jeromekelleher]

I think he means transform all coords in a tree sequence and return the result