Low level infrastructure to handle MS spectra

Externalising the new MS spectra backend-supporting infrastructure from MSnbase. Could either survive as its own package or be phagocyted back into by MSnbase.

Basic ideas and concepts

• Spectra should become the main object to represent MS data.

• No explicit Spectrum objects, spectrum data (m/z - intensity pairs) will be represented as a matrix stored/provided by the Backend class.