[Breaking API Change] Index wav data by sample then channel

Swap indexes of data exposed by wav package. Data is now indexed by sample then channel, e.g.
[sample0[ch0, ch1], sample1[ch0, ch1], ...]

Two reasons for this:
1) Sample is more natural to index on. Data capsulation makes more sense when data for a discrete instance is contained in a discrete structure.

2) Client code is cleaner. Clients get an array of all data per sample rather than iterating through channels and pulling out the sample data.