Move constants into module namespace

These constants are commonly usable.
Merge pull request #17 from FRidh/bands

Move constants into module namespace
Turbulence: Split field generation from spectrum.
Merge pull request #20 from FRidh/turbulence

Turbulence: Split field generation from spectrum.
Reflection: Make boundary class usable.

Up to now the Boundary class was not usable. Now it is possible to
easily create a boundary, calculate the impedance and/or reflection
factor using different models, and plot them.
Merge pull request #21 from FRidh/reflection

Reflection: Make boundary class usable.
Turbulence: Changes to Field plot
Merge pull request #22 from FRidh/turbulence

Turbulence: Changes to Field plot
tox.ini
Patch for tox
Octave: Reference frequency as module constant
Turbulence: Small changes to plotting.
Turbulence: Use autojit

Use autojit to allow faster calculation of the refractive-index field.
Merge pull request #25 from FRidh/dev

Several unrelated commits.
Merge pull request #24 from felipeacsi/master

Tox
Turbulence: Move field generation outside of loop
Merge pull request #26 from FRidh/turbulence

Turbulence: Move field generation outside of loop
Merge branch 'master' of github.com:python-acoustics/python-acoustics into pstd

Conflicts:
	acoustics/turbulence.py
Signal: convolve now has modes similar to numpy and scipy.signal convolve

convolve now supports the modes 'full', 'same' and 'valid' like in
numpy.convolve and scipy.signal.convolve.
PSTD: Support also continuous signals like tones - almost working