deprecate multiValue mode for mapping.Mapping

[GoogleCodeExporter]

We should rationalize Pygr's mapping / graph classes.  Specifically,
Mapping should be clearly restricted to one:one relations; users should use
Graph for *:many relations.  I suggest we deprecate the multiValue option
for Mapping, with a warning message that tells users to use Graph instead.
 For one thing, Mapping's inverse class assumes the original mapping was
one:one.

Original issue reported on code.google.com by cjlee...@gmail.com on 31 Aug 2009 at 9:37