Add conda env file for release v0.8

[Bultako]

This PR adds a conda env file for release v0.8, as discussed in #1725

Another conda env file has been added to gammapy-webpage repo in gammapy/gammapy-webpage#17

In the near future, this conda env file will be also used to create virtual environments for stable versioned releases in Binder, if tutorials are moved to this gammapy repo as proposed in PIG 4 https://github.com/gammapy/gammapy/blob/8ea7f92e758eca32386d468fcdfef479ee567703/docs/development/pigs/pig-004.rst

I would suggest, as a next step, to move environment-dev.yml file to dev folder, and modify documentation accordingly.

[cdeil]

@Bultako - Why duplicate the file in the gammapy and gammapy-webpage repo?
As far as I can see there's no benefit to duplication (and duplication always causes some problems / confusion)

As for the name of the file and environment, you have name: gammapy_v08 and in gammapy/gammapy-webpage#17 also in the filename.
I would suggest to use the version number without dots stripped (0.8), as is e.g. done in the filename gammapy-0.7.tar.gz for source releases: https://pypi.org/project/gammapy/#files
Is that possible, or are dots forbidden in conda env names or filenames used for env yaml files for some reason?

As for where to put the environment-dev.yml - I would suggest to either leave it where it is, or move it with the other conda env files if it's a bit easier to find and maintain there. I think the dev folder at least at the moment mostly contains junk and we might want to delete it some day. There's already many files at the top level that are for devs like Makefile and setup.py and config and CI files, it's not really possible to put them all in a dev folder for a Python project, to get a cleaner top-level directory. Most users will never look at the Gammapy code repo, they will just install Gammapy from elsewhere and look at the docs.

[cdeil]

Why duplicate the file in the gammapy and gammapy-webpage repo?
As far as I can see there's no benefit to duplication (and duplication always causes some problems / confusion)

I forgot to say: my suggestion is that you choose one place where you think maintaining them will be easiest and put them there without duplication. Note that we can add a static build step to the gammapy.org webpage, e.g. if you want to pull in these env files or other files for downlaod / deploy from gammapy.org. If you want to ship those files to users via gammapy.org that doesn't mean they have to be maintained in the gammapy-webpage repo.

[Bultako]

The conda env file will be needed in gammapy repo for versioned envs in Binder, but if shipped via gammapy.org will avoid dependency of Github platform. I also prefer not to have it duplicate, I surely misunderstood what it was kind of agreed in telcon last Friday. So, if I have to choose one place it will be gammapyrepo and keep stable Binder versioned envs, with the naming convention that you suggest.

Re: shipping files to users via gammapy.org
I would like to know how to do this without Github repo. I will need to place some json config files related with gammapy download command for PIG4 as I said in PR gammapy/gammapy-webpage#17

By the way, I guess you can dismiss and close this PR in gammapy-webpagerepo, if we decide to keep the conda env file in gammapy repo.

Re: environment-dev.yml
Ok to leave it where it is.

[cdeil]

OK, so merge this PR, and close gammapy/gammapy-webpage#17 , right?
Go ahead any time.

Re: shipping files to users via gammapy.org I would like to know how to do this without Github repo. I will need to place some json config files related with gammapy download command for PIG4 as I said in PR gammapy/gammapy-webpage#17

gammapy.org will just show the files in this repo: https://github.com/gammapy/gammapy-webpage (in the default gh-pages branch). Any file we want to show up there has to be committed to that repo.
For the data JSON config files, this is OK, no? Just make a data folder in that repo and put whatever files you like there. It's just not a solution to ship large files, but anything up to 10 or 100 MB even would be possible.

[Bultako]

Re: shipping files to users via gammapy.org

I'm saying this because of having a way of getting rid of Github for some JSON config files i.e. what happens if Github disappears, and becase you just said above "If you want to ship those files to users via gammapy.org that doesn't mean they have to be maintained in the gammapy-webpage repo"

For the moment I would like just to put JSON config files, I'll do it in gammapy-webpage repo, data may be still delivered via gammapy-extra repo.

Merging this PR now.