Jp/zernike partials

[ioannisPApapadopoulos]

I managed to implement the partial derivatives with respect to y. The following code now runs:

`using MultivariateOrthogonalPolynomials

W = Weighted(Zernike(1)) Δ = Laplacian(axes(W,1))

∂ʸ = PartialDerivative{2}(axes(W,1))

Z⁰ = Zernike(0) Z¹ = Zernike(1)

∂Y² = (Z¹ \ (∂ʸ Z⁰)) (Z⁰ \ (∂ʸ W)) Δ = Z¹ \ (Laplacian(axes(W,1)) W)

z=2.0; A = (∂Y² - z*Δ); c = A[Block.(1:50), Block.(1:50)] \ rand(1275)`

[codecov[bot]]

Codecov Report

Patch coverage: 95.23% and project coverage change: -0.05 :warning:

Comparison is base (02b1216) 96.21% compared to head (b80a9a4) 96.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #150 +/- ## ========================================== - Coverage 96.21% 96.17% -0.05% ========================================== Files 6 6 Lines 846 888 +42 ========================================== + Hits 814 854 +40 - Misses 32 34 +2 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaApproximation/MultivariateOrthogonalPolynomials.jl/pull/150?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/MultivariateOrthogonalPolynomials.jl](https://app.codecov.io/gh/JuliaApproximation/MultivariateOrthogonalPolynomials.jl/pull/150?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL011bHRpdmFyaWF0ZU9ydGhvZ29uYWxQb2x5bm9taWFscy5qbA==) | `100.00% <ø> (ø)` | | | [src/disk.jl](https://app.codecov.io/gh/JuliaApproximation/MultivariateOrthogonalPolynomials.jl/pull/150?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL2Rpc2suamw=) | `97.16% <95.23%> (-0.48%)` | :arrow_down: |

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[ioannisPApapadopoulos]

Windows tests are failing but that seems to be the case in the main branch as well. Not sure why the code coverage has decreased.. I'm covering the lines that it's complaining about in the tests.

[TSGut]

I've noticed in other packages that the macro @simplify screws with coverage. I don't know why but as far as I've seen those lines never satisfy the coverage checks.