Providing ATLAS with kernel cleanup code

As mentioned in Section 2.1, when any problem dimension (eg., M, N, or K) is not a multiple of $N_B$, ATLAS must call cleanup code to handle the remainder. When the user-contributed kernel is only modestly faster than ATLAS's generated kernel, letting the generated code handle cleanup will probably be an adequate solution. However, when the user-contributed kernel is much faster than the generated code, using the generated cleanup may represent a significant performance drop for many problem sizes (see Section 2.7.5 for an analysis of the cost of cleanup), and thus it becomes necessary for the user to supply ATLAS with cleanup code as well. In order to understand how this is done, it is necessary to discuss how ATLAS does cleanup.