Buffered all BLAS3 output matrices in band_to_band, enable with -DB2B_BUFFER_WRITES. Hopefully will reduce false sharing between OpenMP threads. Strange new valgrind errors from within MKL TRSM on my test platform (emerald), but passes tests; needs to be tested on another platform/valgrind to compare (in other places in this code, emerald's valgrind gives false positives).