Repaired handling of edge cases where there is no useable data to fit.
nans are now returned. The user is alerted with print messages instead
of warnings because Python's default is to show warnings from a given
source line only once. In the case of handling datasets that might not
have any useable values, it is quite possible that a user would run into
empty datasets multiple times during a session, and we want the message
to get to them every time.