Add option to set EQLHarmonic points to constant depth

[santisoler]

Rename the relative_depth parameter for depth and add a new depth_type
parameter. Keep supporting the relative_depth parameter but raise
a FutureWarning if passed. Add a new _build_points() method for creating
the point source coordinates based on the data points. Add test functions for
the new feature using pytest.fixture for sample coordinates. Make examples to
use the new depth argument.

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]

Just a small suggestion to parametrize the test functions. Otherwise looking really good!

[leouieda]

Best to split these tests into multiple ones. Use a fixture for the data and parametrize the options. Otherwise it's hard to tell which part breaks when tests fail.

[santisoler]

Sure!

[santisoler]

I've improved the tests by defining a coordinates fixture and by parametrizing the depth_type and the expected upward array.
I created a new test function for the relative_depth parameters. It will make it easier to deprecate it in the future.

BTW, I used the name="coordinates" while defining the fixture so pylint doesn't complain about redefining it on the test functions. I think we should use that kind of naming scheme all over Fatiando:

• Define fixture functions as fixture_...
• Use the name parameter to set a nice name to it.

Eg:

@pytest.fixture(name="coordinates")
def fixture_coordinates():
    ...

Source: https://stackoverflow.com/questions/46089480/pytest-fixtures-redefining-name-from-outer-scope-pylint