Allow disabling limits

[stephane-caron]

This PR follows from this comment.

[coveralls]

Pull Request Test Coverage Report for Build 10098350690

Details

• 23 of 23 (100.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 99.079%

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Totals
Change from base Build 10098182798:	0.002%
Covered Lines:	1828
Relevant Lines:	1845

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💛 - Coveralls

[stephane-caron]

@domrachev03 @simeon-ned FYI, this PR makes limits similar to tasks. Users will be able to disable them too if they want (by setting limits=[]).

Since we are about to release Pink v3 I'd rather make this API update now. Feel free to share your thoughts.

I also have one question: in barriers there is a compute_qp_inequality but in the rest of the library we call it compute_qp_inequalities. I think we should use the same name for consistency (o/w developers will have to remember which one is singular and which one is plural). I chose "inequalities" before to highlight that there are many of them, but I don't have a strong preference. Do you have an opinion on this?

[simeon-ned]

@stephane-caron I think providing limits as you have done in this PR is a clearer way, also I would prefer to have compute_qp_inequalities as it was before we introduced barriers.