Implement CEL0 algorithm

[Yazhou-Z]

1. Implement the CELO method to the sparsecoding.inference
2. Add a test for the CELO method.
3. Add a jupyter notebook that demonstrates the method working on bars example.

[cshewmake2]

Hi Yazhou!
Awesome job submitting your method and supporting files to the repo! I've made a few comments for correction to address.

The main thing to fix is the update method for CEL0. The update rule is similar to LCA, but it's not quite the same. I've detailed the difference in the inference.py file notes.

I don't know if this is related, but when I run the inference method in the bars notebook example, sometimes coefficients are right, but other times the baseline deviates significantly from zero. I suspect this will be fixed by fixing the CEL0 update rule. I've included an example of the problem below.