scotch5: 5.1.12b - a library for (hyper)graph partitioning

Scotch v5 used to be in homebrew-science, but disappeared in favor of v6. However, v6 has some API incompatibilities, so that some software doesn't work with it. In particular, this applies to the [Mumps package](http://graal.ens-lyon.fr/MUMPS/) (see [this post](https://listes.ens-lyon.fr/sympa/arc/mumps-users/2013-02/msg00002.html) on their mailing list).

I'd like to bring the Mumps package (an excellent sparse linear solver; and also a package in Ubuntu, Debian ...) to homebrew-science, and hence would also need this package.