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]

Codecov Report

❌ Patch coverage is 95.23810% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.17%. Comparing base (02b1216) to head (b80a9a4).
⚠️ Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
src/disk.jl	95.23%	2 Missing ⚠️

Additional details and impacted files

@@            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     

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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.