read_decorr_samples and read_decorr_weights changes

Some changes to make code work with latest version of dart, but mainchange is in unpack_utils, I modified the code to handle signed values. In Java as byteptr was an array of bytes, values were automatically turned into their negative equivalent.

I'm not sure about the change I made to getS16IntFromBytes, but overall with these modifications I was able to successfully decode a small WavPack file.