n-d transforms

[jishnub]

This completes the partially implemented 2D and adds nD transforms, based on Kronecker products of grids.
After this,

julia> x = Fun(); y = Fun()^2;

julia> M = values(x) .* values(y)'
2×3 Matrix{Float64}:
  0.53033   0.0   0.53033
 -0.53033  -0.0  -0.53033

julia> C = transform(Chebyshev() ⊗ Chebyshev(), M)
2×3 Matrix{Float64}:
 0.0  0.0          0.0
 0.5  7.40149e-17  0.5

julia> C ≈ coefficients(x) .* coefficients(y)'
true

julia> itransform(Chebyshev() ⊗ Chebyshev(), C) ≈ M
true

julia> x = Fun(); y = Fun()^2; z = Fun()^3;

julia> M = values(x) .* values(y)' .* reshape(values(z), 1, 1, :);

julia> C = transform(Chebyshev() ⊗ Chebyshev() ⊗ Chebyshev(), M);

julia> C ≈ coefficients(x) .* coefficients(y)' .* reshape(coefficients(z), 1,1,:)
true

[codecov]

Codecov Report

Base: 70.81% // Head: 70.65% // Decreases project coverage by -0.16% ⚠️

Coverage data is based on head (e5963a2) compared to base (c54d5a0).
Patch coverage: 48.83% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   70.81%   70.65%   -0.17%     
==========================================
  Files          77       77              
  Lines        7971     8005      +34     
==========================================
+ Hits         5645     5656      +11     
- Misses       2326     2349      +23     

Impacted Files	Coverage Δ
src/Multivariate/TensorSpace.jl	67.01% <47.61%> (-3.86%)	⬇️
src/Fun.jl	68.94% <100.00%> (ø)
src/Caching/ragged.jl	62.68% <0.00%> (-1.00%)	⬇️

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