Add environment.yml file

[shanosborne]

At the request the JWST Mission Office, INS-created community tools for JWST should meet certain standards for Cycle 1. One of these is details on the software's dependencies in a requirements.txt or environment.yml file.

[shanosborne]

I tested this file by creating an environment with it using the command conda env create -n envname -f environment.yml and all the webbpsf tests passed (provided you have the version of webbpsf that matches the version of poppy in the file, but I think that's a fair requirement and I'm not sure of a way around that)

[coveralls]

Pull Request Test Coverage Report for Build 216

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 43.317%

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Totals
Change from base Build 211:	0.0%
Covered Lines:	2272
Relevant Lines:	5245

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💛 - Coveralls

[mperrin]

As discussed in person, we should revise this to give minimum versions rather than exact versions, I think.

We can also drop six from this and from the setup.py, because that's no longer needed now that we've dropped Python 2 compatibility.

In the dependencies file, why do you list many of the packages as pip installations specifically? I think we can just use conda directly, without invoking pip, to install all of those.

[mperrin]

Also, sphinx is not required to use webbpsf. It's just required to compile the documentation, but we don't expect users to ever do that themselves; they should just look at the page on RTD. So perhaps we don't need to include sphinx in the environment.yml file either?

[mperrin]

Closing, because superseded by #321