Implement sum

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.17% :warning:

Comparison is base (5d0d0f9) 98.33% compared to head (9102cb3) 98.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #44 +/- ## ========================================== - Coverage 98.33% 98.17% -0.17% ========================================== Files 5 5 Lines 541 547 +6 ========================================== + Hits 532 537 +5 - Misses 9 10 +1 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/continuouspolynomial.jl](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2NvbnRpbnVvdXNwb2x5bm9taWFsLmps) | `96.66% <100.00%> (-0.73%)` | :arrow_down: | | [src/piecewisepolynomial.jl](https://app.codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/44?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3BpZWNld2lzZXBvbHlub21pYWwuamw=) | `100.00% <100.00%> (ø)` | |

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