Cartesian product adaptor

[codecov[bot]]

Codecov Report

Base: 96.51% // Head: 96.70% // Increases project coverage by +0.19% :tada:

Coverage data is based on head (a09f834) compared to base (d69aa42). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #10 +/- ## ========================================== + Coverage 96.51% 96.70% +0.19% ========================================== Files 44 46 +2 Lines 1290 1365 +75 ========================================== + Hits 1245 1320 +75 Misses 45 45 ``` | [Impacted Files](https://codecov.io/gh/tcbrindle/libflux/pull/10?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tristan+Brindle) | Coverage Δ | | |---|---|---| | [include/flux/op/cartesian\_product.hpp](https://codecov.io/gh/tcbrindle/libflux/pull/10/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tristan+Brindle#diff-aW5jbHVkZS9mbHV4L29wL2NhcnRlc2lhbl9wcm9kdWN0LmhwcA==) | `100.00% <100.00%> (ø)` | | | [include/flux/op/cartesian\_product\_with.hpp](https://codecov.io/gh/tcbrindle/libflux/pull/10/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tristan+Brindle#diff-aW5jbHVkZS9mbHV4L29wL2NhcnRlc2lhbl9wcm9kdWN0X3dpdGguaHBw) | `100.00% <100.00%> (ø)` | | 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=Tristan+Brindle). 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=Tristan+Brindle)

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