Remove Julia from environment.yml file

[samgdotson]

I couldn't find a guide for contributors, so I hope this is okay. This PR contains no new features but will provide users with a more consistent experience across platforms.

Summary

This PR changes the default environment setup by removing the Julia installation from the environment.yml file. I also updated the section for installing PRAS on Linxu/Mac to install Julia via conda. This change was motivated by Issue #169 and my own experience installing ReEDS with the current documentation.

This change provides the following benefits:

1. A consistent user experience. Since PRAS is optional, Julia should be optional for users on all platforms.
2. ReEDS installation can now be automated on Windows machines, where before it was impossible (or prohibitively difficult).
3. Retains the ability for a single line installation on Unix machines.

conda env create -f environment.yml; conda activate reeds2; conda install -y "julia>=1.8"

4. Reduces additional documentation. My original fix for this was to add instructions to setup.md in the Windows section. This approach is much more streamlined.
    

Technical Details

Issues resolved (if any)

Closes #169

Known incompatibilities (if any)

None

Relevant sources or documentation (if any)

Per this stackoverflow post, Julia is not available on Windows via Conda. It is not clear when this will be changed, if ever.