Use latest tagged version of upstream osqp and osqp-eigen instead of local fork

[traversaro]

Apparently the robotology-superbuild is currently using a fork of osqp for dealing with some .gitattributes problems (see https://github.com/robotology/robotology-superbuild/pull/87/files#diff-fcad91dd1d4b0f9e2cc5f258bfa8b79bR10). As new version of osqp are being released, and sometimes they also contain breaking changes (see robotology/osqp-eigen#31) we should switch to use upstream osqp, optionally on a specific release tag, so that we can update to use new osqp version by just changing the tag.

[traversaro]

The original issue was discussed in #87 (comment) and the following comments. Dropping Ubuntu 16.04 should be sufficient to solve the problem.

[traversaro]

As of #243, the issue seem to be solved, probably due to using the latest ExternalProject module via requiring the latest YCM (see #224 and #199).

[traversaro]

In #260 we used a custom tag for osqp-eigen with our fork of osqp. To fix this we need to update all the software that depends on osqp-eigen in the superbuild to deal with this new version, and then we can update.

@GiulioRomualdi let me know if you need my help on this.

[GiulioRomualdi]

@S-Dafarra check this: #242 (comment) 😄

[traversaro]

Just for clarity, I think the incompatibility for which the fork was necessary was just for Ubuntu 16.04 + GitHub Actions, not Ubuntu 16.04 on its own (that in any case we stopped testing in CI).