Move gmt from requirements.txt to CI scripts instead

[weiji14]

Description of proposed changes

Edit: Simply placing gmt in our Continuous Integration (CI) scripts' CONDA_INSTALL_EXTRA environment variable so that the requirements.txt file remains valid for pip.

Renaming instances of requirements.txt -> conda-requirements.txt, and requirements-dev.txt -> conda-requirements-dev.txt.

The requirements.txt file is a standard file used by pip to install Python packages, but we use it here for installing conda packages?! There are actually some automated bots/services that gets confused by this requirements.txt file, thinking it should use if to install PyPI packages but fails because it's not meant for that.

May want to raise this upstream too at https://github.com/fatiando/continuous-integration.

Needed for Zeit Now continuous documentation PR at #344, may help resolve weiji14/deepbedmap#153

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[seisman]

As far as I can see, all the packages in the requirements.txt and requirements-dev.txt can be installed via pip, except gmt=6.0.0rc4, right? As I said in #257, pygmt doesn't really dependent on the gmt package provided by conda-forge. I removed gmt from the requirements.txt files in #257, but it was added back in #311.

[weiji14]

Yes, the first problem is that the gmt=6.0.0rc4 line in requirements.txt doesn't work in the pip install case.

Second problem is that our CI build actually uses the requirements.txt file to install conda packages. Ideally we should be using an environment.yml (or similar) file to do that...

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[leouieda]

This raises an interesting issue that we'll have to think about. If we publish to PyPI, even though the install would work, the user will get an error on import because GMT hasn't been installed as well. Which means that our PyPI packages are almost useless unless we bundle GMT and other binaries into a wheel (which I'm not looking forward to doing). It would be very frustrating for non-GMT users to pip install pygmt and not get something useful.

I don't have a solution for this. If anyone has some thoughts, that would be great!