Add new `tilt_angle` transformation function

[leomiquelutti]

Add new tilt_angle transformation function that computes the tilt from a given a grid of potential field data. Add the function to the API index. Include tests and an example for the new function.

Changes proposed:

• added tilt function to _transformations.py
• added it to the index.rst
• created an example in tilt.py
• added it to __init__.py
• added the class TestTilt in test_transformations.py

Some points/questions:

• The TestTilt::test_against_synthetic is failing and I don't know why.
• Maybe the plots in tilt.py require a title. I added but had a hard time with pyGMT, so I left them commented.
• I think we should merge the examples, maybe all in one .py file, comparing the transformations from TMI to the ones from RTP TMI. But that should be done in another PR.
• Do I add it to transformations.rst?

Can you please help me @santisoler.

[leomiquelutti]

Do you know why the test for macOS for python 3.8 is failing and/or how to solve it, @leouieda?

[santisoler]

It was due to a failure when trying to push the coverage report to Codecov. I reran it and now it's passing. Nothing wrong with your code ❤️