scotch: Upgrade to 6.0.6.

scotch: Upgrade to 6.0.6.

* gnu/packages/maths.scm (scotch): Upgrade to 6.0.6.
[source]: Remove patches that have been fixed upstream.
[arguments]: Add 'install-metis' phase.
[outputs]: New field.
(pt-scotch, pt-scotch32)[arguments]: Add 'mpi-setup' phase.  Use 'invoke'.
* gnu/packages/patches/scotch-graph-diam-64.patch,
gnu/packages/patches/scotch-graph-induce-type-64.patch,
gnu/packages/patches/scotch-test-threading.patch: Delete files.
* gnu/packages/patches/scotch-integer-declarations.patch: New file.
* gnu/local.mk (dist_patch_DATA): Adjust as necessary.