Abstract selection of coef band.
This patch abstracts the selection of the coefficient band context into a function as a precursor to further experiments with the coefficient context. It also removes the large per TX size coefficient band structures and uses a single matrix for all block sizes within the test function. This may have an impact on quality (results to follow) but is only an intermediate step in the process of redefining the context. Also the quality impact will be larger initially because the default tables will be out of step with the new banding. In particular the 4x4 will in this case only use 7 bands. If needed we can add back block size dependency localized within the function, but this can follow on after the other changes to the definition of the context. Change-Id: Id7009c2f4f9bb1d02b861af85fd8223d4285bde5