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.
Turbulence: Improved plot layout
Travis
Merge pull request #43 from FRidh/travis

Travis: Use conda
Using pytest in Travis
Merge pull request #45 from felipeacsi/travis-numba-tests

Using pytest in Travis
Merge branch 'master' of github.com:python-acoustics/python-acoustics into octave
Merge pull request #41 from FRidh/octave

Octave: several improvements
Re-organizing utils
Fix docs
Merge pull request #38 from felipeacsi/esum

Re-organizing utils