Enhance op, fix bugs and update docstring

[liwt31]

A bunch of misc updates.

[liwt31]

It seems the CircuitCI service is down for no good reason. Relevant link - https://discuss.circleci.com/t/existing-project-now-fails-with-no-configuration-was-found-in-your-project/45097. A ticket is submitted.

Considering switching to GitHub Actions if the problem persists

[codecov[bot]]

Codecov Report

Base: 85.21% // Head: 85.27% // Increases project coverage by +0.05% :tada:

Coverage data is based on head (f56f6e1) compared to base (ed424bd). Patch coverage: 87.50% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #148 +/- ## ========================================== + Coverage 85.21% 85.27% +0.05% ========================================== Files 105 105 Lines 9986 9989 +3 ========================================== + Hits 8510 8518 +8 + Misses 1476 1471 -5 ``` | [Impacted Files](https://codecov.io/gh/shuaigroup/Renormalizer/pull/148?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shuaigroup) | Coverage Δ | | |---|---|---| | [renormalizer/model/op.py](https://codecov.io/gh/shuaigroup/Renormalizer/pull/148?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shuaigroup#diff-cmVub3JtYWxpemVyL21vZGVsL29wLnB5) | `85.16% <87.50%> (+0.70%)` | :arrow_up: | | [renormalizer/mps/mps.py](https://codecov.io/gh/shuaigroup/Renormalizer/pull/148?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shuaigroup#diff-cmVub3JtYWxpemVyL21wcy9tcHMucHk=) | `90.55% <0.00%> (+0.18%)` | :arrow_up: | | [renormalizer/mps/mp.py](https://codecov.io/gh/shuaigroup/Renormalizer/pull/148?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shuaigroup#diff-cmVub3JtYWxpemVyL21wcy9tcC5weQ==) | `88.49% <0.00%> (+0.30%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shuaigroup). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=shuaigroup)

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[liwt31]

The CI service is back but the result is based on my own repo. Feels weird but no harm for the moment. I prefer keeping the GitHub actions CI since it's free anyway.