Octave: several improvements

- Let certain methods call functions instead of duplicating their logic.
Include references to these functions in the docstrings.
- lower_frequency and upper_frequency now also have order=1 by default.
Merge branch 'master' of github.com:python-acoustics/python-acoustics into octave
Merge pull request #41 from FRidh/octave

Octave: several improvements