Reduce envrionment.yml dependencies

[terhorstd]

The definition of specific builds in the dependencies is dangerous and leads to problems when packages move on. The environment file should only contain top level dependencies that are explicitly required. These dependencies may pull in deeper ones if they need, but it's not for this env to define which that should be. Also the dependencies should only specify to the actual requirements, i.e. probably it is not necessary to have Python "3.7.3=h0371630_0" but any "3.7" would work, maybe even any "3".
This is aimed to fix #3 but I don't have a Mac and can't try. Maybe @jakobj could also have a look if there is more unnecessary dependencies to cut.

[bhargavCSSE]

Thanks for reviewing the issue. I managed to set it up on ubuntu with no problem. However, I do agree with you on your point that the environment file should only contain top level dependencies that are explicitly required.

[bhargavCSSE]

Excellent work on building this tool btw 👍

[terhorstd]

@bhargavCSSE, you could try to remove the gcc-ng dependency on Mac to see if it's really needed. If not, then it should not be in the dependencies. The most clean way would actually be to start with a clean/minimalistic environment.yml and just add lines from the current one if something doesn't work. I would do that, but as mentioned don't have access to a Mac.
If you can get it to run with minimalistic environment, I'd be interested to add your experience to this PR and merge it soon, so you can work with master again.

[terhorstd]

Excellent work on building this tool btw +1

That goes to @jakobj and @weidel-p
😉

[bhargavCSSE]

I believe libgcc-ng is needed to compile MPI libraries. Let me work on it and see how it goes. Thanks :)

[bhargavCSSE]

This still does not work with macOS. No matter though. Its better to work with NEST on ubuntu :). BTW, the new environment file looks much better even for ubuntu platform.