Fix octave band filters and their bandwidths

[fakufaku]

This PR fixes several problems I noticed.

1. The lowest octave band filter was malformed when using "octave_bands_keep_dc=True".
2. The bandwidth of the octave bands were not correct for the lowest and high bands.
3. Refactor the way the RIR is weighted with the histogram in simulation/rt.py.
4. Adds a test that compares the energy decay of ISM vs RT.
5. Improves the implementation of the RT60 measurement function.

• 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 ?
• 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".