clean up and rework packaging for official Ubuntu release

[berndpfrommer]

With libmetis about to be cleanly decoupled from GTSAM (borglab/gtsam#570) I would like to redo the debian packaging with an eye towards an official ubuntu release.

While I don't know what this exactly entails here is a TODO list (to be expanded upon):

• make sure packaging follows standards for shared libs (https://www.debian.org/doc/debian-policy/ch-sharedlibs.html)
• build with system libmetis
• add dependency on system libmetis
• getting rid of all warnings that are currently coming from the packaging tools
• running the right triggers after installation: trigger ldconfig after installing gtsam #16

Right now the only "release" package we have is a ppa that releases 4.0.3.

Questions (@jlblancoc @acxz @ProfFan):

• when will 4.1.0 be released? I see a release candidate (https://github.com/borglab/gtsam/releases) tag for more than a year, and there even is a release branch, but maybe I missed the official announcement.
• the removal of libmetis is not even in the development branch yet (probably soon), but is necessary for a first clean release. The branch release/4.1.0 obviously does not have those commits yet. Should we try to back port those changes, or is there a new release coming up soon that will come off the development branch and therefore will have those commits?

For me the most convenient way to keep official release packaging sane would be to have a tag e.g "4.x.0" at the point where the release branch "release/4.x.0" comes off the "develop" branch.

Note that once 4.1.0 is released as an official ubuntu package we cannot break the binary compatibility (ABI) without bumping the major version number (to e.g. 5.x). I need to look into tools for checking that automatically (https://qt-kde-team.pages.debian.net/symbolfiles.html)

[berndpfrommer]

The debian package is now using the system provided libmetis:

borglab-launchpad/gtsam-packaging@d14daa7