Ruby/Python bindings packaging

[j-rivero]

A bit of jazz in the PR for different things:

• Remove Sid: we already update ign-math6 in Debian unstable (aka Sid) and the become ready for users. Not need to build our owns.
• Add support for ruby bindings: build-dep need ruby-dev and the new ruby-ignition-math6 package is ready
• Add support for python bindings: python helper via dh_python and the new package python3-ignition-math6
• Added long description to math6 packages

Tested here: https://build.osrfoundation.org/job/ign-math6-debbuilder/1029/

[chapulina]

Python packages LGTM, but we need to set PYTHONPATH=/usr/lib/x86_64-linux-gnu/python before using, I don't know if that's standard.

I haven't checked the Ruby packages, but I can try next week.

[chapulina]

Python packages are working for me without setting new env vars as of the latest nightly 🎉

For Ruby, I still need to set an env var manually:

$ RUBYLIB=/usr/lib/x86_64-linux-gnu/ruby/ irb
irb(main):001:0> require 'ignition/math'
=> true