[WIP] Add an `environment.yml` for `conda`-users

[cjayb]

This works on macOS and Ubuntu 16.04. If included, it would reduce our installation instructions to

conda create --file environment.yml

The modification to setup.cfg doesn't influence the behaviour of python setup.py bdist_wheel, though: a universal wheel is still built (ping #255)

[jasmainak]

I think a conda route is fine but let's try to make the pip route simpler first. Example by modifying the setup.py as in #255

For example, what would a user do if they wanted to use both MNE and HNN? Environment files cause problematic workflows and make people too dependent on specific versions of their software for their results.

[cjayb]

There's this:

conda env update -f other_env.yml

I guess the whole point of conda is to ease the pain of version management. For pure-python pip is preferable, but as our wheels-experiment is revealing, c-extensions are a real b****!

I can live with trying to work out the wheels though, and thinking of conda-forge as a last resort...

I'll close this PR, nothing here that can't be reinvented later :)

[jasmainak]

I think this is all good! Let's keep this open for now while we close the documentation PRs and bring hnn-core closer to release

[rythorpe]

I'm revitalizing this PR in conjunction with #501 in order to get hnn-core running on Binder. Is it cool if I take over @cjayb?

[jasmainak]

@rythorpe I don't think we need this PR for binder (see here) but a conda-based install would be cool! Specially if it simplifies installing the parallel backends