Support Ubuntu 20.04 and release 1.0.7

[traversaro]

This PR fixes some dependencies as installed in Ubuntu 20.04 :

• Assimp, by adding a workaround for Assimp configuration fail on Ubuntu 20.04 #693
• Octave, backporting the fixes from https://github.com/robotology/idyntree/pull/677\

It also add support for Ubuntu 20.04 compilation in GitHub Actions, disabling for now the tests as some tests are failing on Ubuntu 20.04, see #694 .
It also bumps the version to 1.0.7, as the idea is to do a last release in the 1.0 series compatible with Ubuntu 20.04, and then do the 1.1 release by merging devel in master.

[traversaro]

For some reason on Ubuntu 20.04 is also necessary to install libgfortran-10-dev . Actually that was just a cache collision problem, in which the cache of Ubuntu 18.04 was used for Ubuntu 20.04. I added the ${{ matrix.os }} string in the cache key to avoid the problem (there was already runner.os, but probably for some reason that does not change).

[traversaro]

There is still a problem when compiling examples with IDYNTREE_USES_KDL, as now console_bridge exports exported targets.