Fixes #191 a bug when setting air absorption coefficients manually

This PR fixes #191

[X] Are there docstrings ? Do they follow the numpydoc style ?
[X] Have you run the tests by doing nosetests or py.test at the root of the repo ?
[X] 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.
[X] 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.
[X] Last but not least, did you document the proposed change in the CHANGELOG file ? It should go under "Unreleased".

Happy PR :smiley:

LCAV / pyroomacoustics