Add tilt transformation

[leomiquelutti]

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.