When TempoMap::copy_points() is called, the new points are intended to belong
to the (nascent) new map. But the copy constructor for the points leaves the
_map member of a Point unchanged, and so the new points reference the old
map (forever!). ::copy_points() must reset each Point to reference the new map.
Refactored the object that has the _map member, so that we could limit access
to its ::set_map() method to TempoMap.
f60b35483d