Simplify interfaces

Eliminated duplicated returns of the type ("Clean up func sigs")
func foo(dst []float64, ...) []float64 { ... }
where the returned slice was also dst.  This makes the fact that those functions operate via side effect more explicit, and will prevent callers from redundantly assigning the result to a new slice.

I also cleared up a few edge cases in the code where zero length slices were provided to the functions.

The docs were cleaned up and variable names were made more uniform.

Testing has 100% coverage at this time.

What do you think of the changes?
@kortschak  any comments on what is idiomatic here? The original is probably faster, but that'll go away over time.