Orientation parameter for RealSphericalHarmonicsDirectivity module.

[je-fleischhauer]

I added an orientation parameter to the RealSphericalHarmonicsDirectivity module. I created a unit test to check that the orientation parameter works, and updated the CHANGELOG file.

• [] Are there docstrings ? Do they follow the numpydoc style ?
• [] Have you run the tests by doing nosetests or py.test at the root of the repo ?
• [] Have you checked that the doc builds properly and that any new file has been added to the repo ? How to do that is covered in the documentation.
• [] Is there a unit test for the proposed code modification ? If the PR addresses an issue, the test should make sure the issue is fixed.
• [] Last but not least, did you document the proposed change in the CHANGELOG file ? It should go under "Unreleased".

Happy PR 😃

[fakufaku]

Thanks for the contribution!