Replace conda for pip on Actions and split requirements files

[santisoler]

Create a new env directory with some requirements-*.txt files, each one
with the specific dependencies to run certain tasks, like checking style,
running the test, publishing to PyPI and building the docs. The GitHub Actions
now use pip to install the dependencies instead of having to setup Miniconda
and install them through conda. The only exception is the one that builds the
docs: we need pyproj and cartopy to do so and they are easier to install
through conda.

Fixes #257

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 and the base __init__.py file for the package.
• Write detailed docstrings for all functions/classes/methods. It often helps to design better code if you write the docstrings first.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.
• Add your full name, affiliation, and ORCID (optional) to the AUTHORS.md file (if you haven't already) in case you'd like to be listed as an author on the Zenodo archive of the next release.

[leouieda]

Looks good to me! Left a comment on the pip cache part but I'm not sure if it's relevant.