Remove exact eigen requirement

[Tobias-Fischer]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

See #19 for reasoning. Basically ceres requires that Eigen is unnecessarily the exact same version when compiling downstream pacakges. Fix #19

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[Tobias-Fischer]

@conda-forge-admin, please rerender

[Tobias-Fischer]

/cc @traversaro

[github-actions]

Hi! This is the friendly automated conda-forge-webservice.
I tried to rerender for you, but it looks like there was nothing to do.

[jschueller]

I'm not too sure on this: upstream added this for a reason, why not ask them if it can be removed ?

[traversaro]

I'm not too sure on this: upstream added this for a reason, why not ask them if it can be removed ?

This is a good idea, I can open an issue upstream later today (CEST).

[Tobias-Fischer]

Checking with upstream is probably a good idea. In this case, while we wait to hear back, shall we merge in #22 so we resolve issues downstream in the meantime?