Move dictionary learning code into module

[jakirkham]

The dictionary learning code has been living in the notebook for sometime to make it easy to use the latest Jupyter Notebook with the 2.x release branch. However that makes it harder for people to understand the notebook as there is a lot of code to parse. As the intent was always to move this code into a module, we can do this and then make a 3.0.0 release with these changes. That way compatibility with the 2.x branch can be ignored going forward.

[jakirkham]

Should add that we need to get rid of sharedict as it is being deprecated.