refactor: move optimization out of init and into fit (#170)

* style: per sklearn, attributes learned from data end with underscore

* refactor: move optimization out of init and into fit

* fix: use new fit() for test

* style: end fit() with return self

* docs: update docstring to reflect new functionality

* refactor: use rho and eta in fit

* feat: use reset for sequential refinements

* fix: use copies for safety

---------

Co-authored-by: John Halloran <jhalloran@oxy.edu>

Author: john-halloran

src/diffpy/snmf/main.py

@@ -15,8 +15,9 @@
     init_stretch=init_stretch_file,
     show_plots=True,
 )
+my_model.fit(rho=1e12, eta=610)
 
 print("Done")
-np.savetxt("my_norm_components.txt", my_model.components, fmt="%.6g", delimiter=" ")
-np.savetxt("my_norm_weights.txt", my_model.weights, fmt="%.6g", delimiter=" ")
-np.savetxt("my_norm_stretch.txt", my_model.stretch, fmt="%.6g", delimiter=" ")
+np.savetxt("my_norm_components.txt", my_model.components_, fmt="%.6g", delimiter=" ")
+np.savetxt("my_norm_weights.txt", my_model.weights_, fmt="%.6g", delimiter=" ")
+np.savetxt("my_norm_stretch.txt", my_model.stretch_, fmt="%.6g", delimiter=" ")