no defaults channel in environment.yaml

[fneum]

Fixes #142
Let's see...

[coroa]

Channel pinning should be superseeded by setting channel priority to strict as explained here: https://conda-forge.org/docs/user/tipsandtricks.html

Apart from that, it seems descartes is pulled in too late by pip and should probably be uplifted to a conda dependency instead.

[fneum]

Pity there's not yet a way to request strict channel priority in the environment.yaml: conda/conda#8675

conda config --set channel_priority strict

[coroa]

Failure on mac:

ImportError: dlopen(/Users/travis/miniconda/envs/pypsa-eur/lib/python3.7/site-packages/fiona/ogrext.cpython-37m-darwin.so, 2): Library not loaded: @rpath/libwebp.7.dylib

  Referenced from: /Users/travis/miniconda/envs/pypsa-eur/lib/libtiff.5.dylib

  Reason: Incompatible library version: libtiff.5.dylib requires version 9.0.0 or later, but libwebp.7.dylib provides version 8.0.0

Both are from conda-forge:

libtiff                   4.1.0                h2ae36a8_5    conda-forge
libwebp                   1.0.2                hd3bf737_5    conda-forge
libwebp-base              1.1.0                         2    conda-forge

Should be missing deps in the libtiff or libwebp conda recipes.

Found conda-forge/libtiff-feedstock#56 and conda-forge/libwebp-feedstock#26 so seems to be a known active problem. We maybe could constrain libwebp down a bit, but this will be a pain to find out, if we can only test on the travis mac runners.

[fneum]

@FabianHofmann everything seems to work with/without defaults channel. I'm on the fence about keeping or removing it. What's your view?