add requirements.txt

[evanbiederstedt]

I'm using the master branch, but feel free to use a different branch.

Related to: https://github.com/hubmapconsortium/vitessce-data/issues/111

• Add a requirements.txt for pip installation.
• Notes in the README

There were two issues I came across:

• I changed the version of scikit-image, as I was getting the following error:

  ERROR: Could not find a version that satisfies the requirement scikit-image==0.16.1 (from -r requirements.txt (line 20)) (from versions: 0.7.2, 0.8.0, 0.8.1, 0.8.2, 0.9.0, 0.9.1, 0.9.3, 0.10.0, 0.10.1, 0.11.2, 0.11.3, 0.12.0, 0.12.1, 0.12.2, 0.12.3, 0.13.0, 0.13.1, 0.14.0, 0.14.1, 0.14.2, 0.14.3, 0.14.5, 0.15.0, 0.16.2, 0.17.1, 0.17.2)

• The installation for obonet runs into issues via wheezy.template. This appears to be a problem noted elsewhere. It sticks on the following:

  Collecting wheezy.template
  Using cached wheezy.template-0.1.195.tar.gz (17 kB)

Review request: @ilan-gold @keller-mark