Get/Set number of threads of specific BLAS/LAPACK library (when there are multiple)

[carstenbauer]

IIUC, when there are multiple BLAS / LAPACK libraries stacked on top of each other, the lbt_get_num_threads / lbt_set_num_threads functions currently try to query or set the number of threads of all libraries. I wonder whether there should be a way (API) to get/set the number of threads of a particular library in the stack using the LBT-registered getter and setter functions, e.g. lbt_get_num_threads(libindex) and lbt_set_num_threads(nthreads, libindex).

Triggered by this discussion, where we stack BLIS (for BLAS) on top of OpenBLAS (for LAPACK). In this case, it might be useful to independently set the number of threads for BLIS and OpenBLAS (even more so in the future, where we might provide a LAPACK which uses the BLIS BLAS under the hood).

Thoughts?