rename libmetis -> libmetis-gtsam to avoid collision with system libs

[berndpfrommer]

At the moment GTSAM builds its own private libmetis.so from the sources in gtsam/3rdparty/metis. This creates a single-architecture libmetis.so file without ABI number, which breaks the building of a multi-architecture Debian package because GTSAM's libmetis.so file collides with the one from the standard libmetis5 Debian package.

The best fix is probably to first make use of 3rdparty/metis optional (see #309 and #292), as was done already for 3rdparty/Eigen, and ultimately directly rely on libmetis5-dev for development of GTSAM.

Until such more elaborate fixes are in place, this PR renames the 3rdparty libmetis to libmetis-gtsam to make Debian/Ubuntu multi-arch builds possible, and avoid accidentally compiling against headers from 3rdparty/metis, but linking against libmetis.so from libmetis5.

Referencing @jlblancoc for review.

---

This change is 

[jlblancoc]

LGTM.

Will probably need to be iterated after testing: we need to test building a gtsam user program built agains the .deb produced by this, to check that none of the cmake exported targets are missing, etc.

In fact, it would be great to add a new Unit Test consisting of invoking cmake to build a simple program using the just-generated gtsam*-config.cmake + gtsam*-targets.cmake, etc. I'll file a separate wish-list issue ticket for it.