Variety of changes, preparing for separate repair code release

[matthewsot]

This PR contains a variety of housekeeping updates. The primary change is that we are refactoring the repair code into a separate repository for the relevant PLDI '21 paper.

Full summary of changes:

• Update bazel-python, which now comes built-in with a lot of the features we were adding manually here (such as a script to generate coverage reports).
• Remove the repair and masking-network code, replacing them with stubs to alert anyone still relying on them that that code has been removed and options they have to use older versions.
• Add some extra options and methods to the different DNN layer classes in PySyReNN. These are used by the new repair code.
• Update Pillow to 8.1.1 as suggested by Bump pillow from 7.1.0 to 8.1.1 #8
• Speed up the BMC code and add code to do BMC with Marabou, ERAN
• Tag last v1.0 release.

Still todo:

• Update the README

[adityathakur]

@zhe-tao PTAL

[zhe-tao]

LGTM.

[adityathakur]

LGTM