Reduce input/output buffer copies in model processing

[mikeoliphant]

Currently, there are at least two buffer copies (one at input and one at output) that could be avoided by directly using the passed in input/output buffers.

[olilarkin]

I agree. Also, the whole allocating arrays of raw input/output pointers stuff in both NeuralAmpModelerCore and the Plug-in code is very suspicious and possibly the source of some of the memory leaks.