Support variable coefficients

[codecov[bot]]

Codecov Report

Base: 86.82% // Head: 98.02% // Increases project coverage by +11.20% :tada:

Coverage data is based on head (79d7c2f) compared to base (8818251). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #14 +/- ## =========================================== + Coverage 86.82% 98.02% +11.20% =========================================== Files 1 1 Lines 129 152 +23 =========================================== + Hits 112 149 +37 + Misses 17 3 -14 ``` | [Impacted Files](https://codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/14?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/PiecewiseOrthogonalPolynomials.jl](https://codecov.io/gh/JuliaApproximation/PiecewiseOrthogonalPolynomials.jl/pull/14/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1BpZWNld2lzZU9ydGhvZ29uYWxQb2x5bm9taWFscy5qbA==) | `98.02% <100.00%> (+11.20%)` | :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=JuliaApproximation). 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=JuliaApproximation)

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