Refactor of ckconv.nn APIs + demo notebook for Arxiv paper

[rjbruin]

Major changes

• APIs of CKConv and FlexConv now take parameters (instead of ConfigDicts) and have default values, for ease of use.
• Added demo notebooks, to showcase usage of FlexConv.
• Added testcases: use testcase.save & testcase.load to save/load a string of training losses to/from file, as a fingerprint for the training run. When loading, if the fingerprint doesn't match, the testcase raises an AssertionError. We use this to verify that any changed code does not change the training behavior.
  • Specifically, I implemented and used this to verify that the other listed changes do not affect the reproducibility of the paper's experiments with this codebase.

Minor changes

• regularize_gabornet()s arguments were trimmed.

[rjbruin]

David let me know he doesn't have time to review, but trusts me. :)