Additional cleanups in fit.py

Additional cleanups in fit.py

* Added outqueue to tests, which turned up an 'outqeue' typo.
* Store info in .set_data().  This allows subclasses to write
  data-specific information when the data changes.
* .guess_scale() returns Null by default.  This tells leastsq to use
  its internal scale estimating magic.
* Added strictly positive scale assertion.  The error message returned
  by leastsq for zero or negative scales isn't very obvious.