Make adding sets a union operation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 94.38%. Comparing base (fc25182) to head (1aa564d).

Files	Patch %	Lines
src/pyoframe/constraints.py	83.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #50 +/- ## ========================================== - Coverage 94.42% 94.38% -0.05% ========================================== Files 14 14 Lines 1309 1317 +8 ========================================== + Hits 1236 1243 +7 - Misses 73 74 +1 ``` | [Flag](https://app.codecov.io/gh/Bravos-Power/pyoframe/pull/50/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bravos-Power) | Coverage Δ | | |---|---|---| | [smart-tests](https://app.codecov.io/gh/Bravos-Power/pyoframe/pull/50/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bravos-Power) | `94.38% <90.90%> (-0.05%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Bravos-Power#carryforward-flags-in-the-pull-request-comment) to find out more.

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