Fix multiplication of MultivariateFun

[jishnub]

After this

julia> f = (x,y)->x^2 * y^3
#983 (generic function with 1 method)

julia> L = LowRankFun(f, Chebyshev() ⊗ Chebyshev())
LowRankFun on Chebyshev()⊗Chebyshev() of rank 1.

julia> (L^2)(0.1, 0.2) ≈ (L * L)(0.1, 0.2)
true

julia> P = ProductFun(f, Chebyshev()⊗Chebyshev())
ProductFun on Chebyshev()⊗Chebyshev()

julia> (L * P)(0.1, 0.2) ≈ (P * L)(0.1, 0.2) ≈ (L^2)(0.1, 0.2)
true

[codecov]

Codecov Report

Base: 69.40% // Head: 71.14% // Increases project coverage by +1.73% 🎉

Coverage data is based on head (8f9f88a) compared to base (b3f1a79).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   69.40%   71.14%   +1.73%     
==========================================
  Files          77       77              
  Lines        8007     8015       +8     
==========================================
+ Hits         5557     5702     +145     
+ Misses       2450     2313     -137     

Impacted Files	Coverage Δ
src/Multivariate/LowRankFun.jl	66.01% <0.00%> (+1.84%)	⬆️
src/Multivariate/Multivariate.jl	34.00% <0.00%> (+0.66%)	⬆️
src/Multivariate/ProductFun.jl	37.50% <0.00%> (+14.08%)	⬆️
src/Operators/general/algebra.jl	91.34% <0.00%> (+0.29%)	⬆️
src/Fun.jl	69.28% <0.00%> (+0.34%)	⬆️
src/LinearAlgebra/clenshaw.jl	39.47% <0.00%> (+0.52%)	⬆️
src/Caching/blockbanded.jl	81.67% <0.00%> (+0.76%)	⬆️
src/Spaces/DiracSpace.jl	71.18% <0.00%> (+0.84%)	⬆️
src/Operators/general/CachedOperator.jl	66.10% <0.00%> (+1.69%)	⬆️
src/Space.jl	86.93% <0.00%> (+1.80%)	⬆️
... and 21 more

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