Integer -> Floats

At many places integers were used while floats should be used. Often
this works fine but not always, resulting in hard to find errors.

Changes:

- Integer -> Float in many places
- SOUNDSPEED module constant
- NC curves are now in a dictionary.
Turbulence: Improved plot layout
Travis
Using pytest in Travis
Merge branch 'master' of github.com:python-acoustics/python-acoustics into room

Conflicts:
	acoustics/core/descriptors.py
Merge pull request #44 from FRidh/room

Integer -> Floats