Remove error and add a warning for gravmag.transform.upcontinue when height <= 0

[rafaelmds]

Fixes #235

Change the behavior of gravmag.transform.upcontinue. Now, instead of an error its gives a warnings.warn when height <= 0.
This change is mainly for educational purposes (you can test what happen if you calculate for it for height <= 0).

Checklist:

• Make tests for new code (at least 80% coverage)
• Create/update docstrings
• Include relevant equations and citations in docstrings
• Docstrings follow the style conventions
• Code follows PEP8 style conventions
• Code and docs have been spellchecked
• Include new dependencies in doc/install.rst, environment.yml, ci/requirements-conda.txt and ci/requirements-pip.txt.
• Documentation builds properly (run cd doc; make locally)
• Changelog entry (leave for last to avoid conflicts)

[leouieda]

@rafaelmds thanks for the PR! Could you please add the following code to fatiando/gravmag/tests/test_transform.py? The function should go below the test_upcontinue function and the import at the top. This will test if the warning is being raised properly.

import pytest


def test_upcontinue_warning():
    "gravmag.transform upward continuation raises warning if height <= 0"
    model = [Prism(-1000, 1000,-500, 500, 0, 1000, {'density': 1000})]
    shape = (100, 100)
    x, y, z = gridder.regular([-5000, 5000, -5000, 5000], shape, z=-500)
    data = prism.gz(x, y, z, model)
    with pytest.warns(UserWarning):
        up = transform.upcontinue(x, y, data, shape, height=0)
    with pytest.warns(UserWarning):
        up = transform.upcontinue(x, y, data, shape, height=-100)

[rafaelmds]

@leouieda Sure! I thought that was necessary but I wasn't sure about it.

[leouieda]

No problem. This is the part that always gets newcomers.

Could you also please change the warning message to "Using 'height' <= 0 means downward continuation, which is known to be unstable." That might be more informative.

[leouieda]

After that, you can add the changelog entry for this PR in doc/changelog.rst and you're done 👍

[leouieda]

@rafaelmds merged! Thanks