conversion/maxspace/union rules for normalized spaces

[jishnub]

After this, the following is type-inferred:

julia> @inferred Fun(NormalizedLegendre()) + Fun(NormalizedLegendre())
Fun(NormalizedLegendre(), [0.0, 1.63299])

The idea is that since polynomial spaces may be converted to each other, the same should also hold for normalized polynomial spaces.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09% 🎉

Comparison is base (c48fd6f) 72.37% compared to head (eeb17b1) 72.46%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
+ Coverage   72.37%   72.46%   +0.09%     
==========================================
  Files          29       29              
  Lines        2744     2746       +2     
==========================================
+ Hits         1986     1990       +4     
+ Misses        758      756       -2     

Files Changed	Coverage Δ
src/Spaces/Jacobi/JacobiOperators.jl	86.21% <100.00%> (+0.77%)	⬆️
src/Spaces/PolynomialSpace.jl	93.52% <100.00%> (+0.11%)	⬆️

... and 3 files with indirect coverage changes

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