Absorb concept of volume_layer array into ocean_model.
Still need to resolve a few issues where this breaks volume reverberation.
Marked those instances @todo tags.
Replace all #include guards in usml::ocean package with #pragma once.
Although it is a little less standard, it is much less error prone.
Bring back matrix version of scattering() method in scattering_model.
Add version of volume_flat constructor that takes scattering model.
Improve documentation of interface vs. volume reverb.

Issue: Cleanup usml::ocean API #64