Set the size of the map. This MUST be called before addMapping().
It is assumed (but not verified) that addMapping() will then be
called exactly 'size' times, with different origOrdinals between 0
Return the map from the taxonomy's original (consecutive) ordinals
to the new taxonomy's ordinals. If the map has to be read from disk
and ordered appropriately, it is done when getMap() is called.
getMap() should only be called once, and only when the map is actually
needed. Calling it will also free all resources that the map might
be holding (such as temporary disk space), other than the returned int.