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?
I've been thinking that this and Fill should go away. It encourages people to create closures in order to save a couple of characters. This makes for less legible code. I'd rather people us a for loop than Fill and Apply. Thoughts?