robustifies the RT60 computation function

[fakufaku]

This modification adds a new parameter to the measure_rt60 routine that allows to discard long noisy tails in measured RIR.

• 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 😃