Fixing installation issues

[jrfarah]

We address issues with installing SNEx2 on a variety of machines. We documented several issues that needed fixing, including:

• Dependency problems
• Github configuration issues
• Filesystem setup problems
• Dockerfile issues

We implement the following fixes to address these issues and more:

• Adding specific gevent version to requirements.txt
• Modifying Dockerfile environment version to python3.9-slim
• Upgrading pip via the Dockerfile
• Removing ligo.skymap from requirements.txtand reinstalling directly via the Dockerfile
• Adding specific platform request to docker-compose.yml

The installation procedure which takes advantage of these changes is available in the SNEx2 documentation (contact @crpellegrino for access).

Thanks to @gterreran , @cmccully , @crpellegrino for help!

[jrfarah]

Moved to #15