BUG: fix the nan_to_nat edge
BUG: fix the nan_to_nat edge

This edge would get called with non float arguments which caused a
typeerror to be raised. This is now caught and a notimplemented error is
raised so that convert can work around this.

cowlicks/odo,cpcloud/odo,quantopian/odo,cowlicks/odo,cpcloud/odo,ContinuumIO/odo,blaze/odo,blaze/odo,ContinuumIO/odo,quantopian/odo