Upgrade lucid to use tensorflow v2 APIs and make it work on Colab platform

[ercaronte]

This PR is to update the lucid repository so that it is possible to use it in Colab, now that it does not longer support the magic command %tensorflow_version 1.x. I have updated the python library python code and most of the notebooks. Since the lucid gs bucket is down, I have also fixed some of the links to static content, pointing temporarely to my lucid fork. If the PR is accepted, it will be possible to update those links to point to the official repo. As a future work to maintain this repo:

• upgrade a handful of notebooks in the misc directory,
• upgrade the opengl code so that is works in kaggle,
• complete the migration to tensorflow v2 to remove any dependency to the tf.compat.v1 code.

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[ercaronte]

While this is being handled by the lucid team, I have merged these changes on my fork https://github.com/ercaronte/lucid.

For anybody willing to use the lucid library on Colab and Kaggle feel free to use it by replacing the lucid package install with: !pip install --quiet git+https://github.com/ercaronte/lucid.git

[smith-kyle]

A bit of an old PR but I noticed that the ReviewNB link is perma-loading. These notebook changes are reviewable in GitNotebooks: https://gitnotebooks.com/tensorflow/lucid/pull/305