Using the official C interface to BLAS/LAPACK

[mohawk2]

The official C interface to LAPACK is LAPACKE, user guide at https://netlib.org/lapack/lapacke.html - we'll want to use the "mid-level" interface, so we can pass in work arrays.

The official C interface to BLAS is CBLAS, guide at https://github.com/Reference-LAPACK/lapack/blob/master/DOCS/CBLAS.md - we'll want to use the _64 variants so that all integers will be 64-bit (which is the "ILP64" model, see https://en.wikipedia.org/wiki/64-bit_computing).

It will be useful to add doc notes for packages users can install to get the headers etc they need. On Debian, that can be listed with apt-file search cblas.h and apt-file search lapacke.h.

Executing this should solve #15.