This adds methods to compute bivariate Gram matrices.

[MikaelSlevinsky]

The test shows how to put together the Gram matrix for OPs with respect to $w(x,y) = 1+|x|+|y|-|xy|$ as a sum of four separable weights.

The complexity scales like O(b^2n^2) for the construction of banded-block-banded Gram matrices (with bivariate moments that have b+1 nonzero blocks). The banded structures can be encoded by making a PaddedArray out of a finite number of moments.

Certains parts of the code run into snags from viewing blocks of banded-block-banded matrices. Could be cleaned up if JuliaLinearAlgebra/BlockBandedMatrices.jl#223 and JuliaLinearAlgebra/BandedMatrices.jl#474 are addressed @dlfivefifty.

Bivariate Jacobi matrices can be found and computed elsewhere (e.g. MultivariateOrthogonalPolynomials.jl).

[MikaelSlevinsky]

P.S. I think the banded-block-banded complexity above is better than 2D Clenshaw. Is that O(b^4n^2)?

In 1D, Clenshaw costs O(b^2n), or O(n^3) if b = O(n), so I think in 2D it must scale worse than O(b^2n^2), either O(b^3n^2) or O(b^4n^2).

[codecov]

Codecov Report

Attention: Patch coverage is 98.16092% with 8 lines in your changes missing coverage. Please review.

Project coverage is 79.80%. Comparing base (2446237) to head (688994c).

Files with missing lines	Patch %	Lines
src/BivariateGramMatrix.jl	98.15%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #261      +/-   ##
==========================================
+ Coverage   77.32%   79.80%   +2.48%     
==========================================
  Files          17       18       +1     
  Lines        3206     3640     +434     
==========================================
+ Hits         2479     2905     +426     
- Misses        727      735       +8     

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