Operator constructor instead of convert

[jishnub]

This is a somewhat large PR, but the idea is the same across files: define the constructor Operator{T}(x::Operator) and the constructor (::Type{T})(x:Operator) where {T<:Operator}, where T and typeof(x) only differ in some type parameter.

After this, something like this would work:

julia> D = Derivative(Chebyshev())
ConcreteDerivative : Chebyshev() → Ultraspherical(1)
 ⋅  1.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅   2.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅   3.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅   4.0   ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅   5.0   ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅   6.0   ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   7.0   ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   8.0   ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   9.0  ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋱
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋱

julia> typeof(D)(D)
ConcreteDerivative : Chebyshev() → Ultraspherical(1)
 ⋅  1.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅   2.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅   3.0   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅   4.0   ⋅    ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅   5.0   ⋅    ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅   6.0   ⋅    ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   7.0   ⋅    ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   8.0   ⋅   ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   9.0  ⋅
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋱
 ⋅   ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅    ⋅   ⋱

The motivation behind this is to get something like the following to work:

julia> C1 = Conversion(Chebyshev(), Ultraspherical(1));

julia> C2 = Conversion(Chebyshev(), Ultraspherical(1.0));

julia> push!([C1], C2)
2-element Vector{ApproxFunBase.ConcreteConversion{Chebyshev{ChebyshevInterval{Float64}, Float64}, Ultraspherical{Int64, ChebyshevInterval{Float64}, Float64}, Float64}}:
 ConcreteConversion : Chebyshev() → Ultraspherical(1)
 ConcreteConversion : Chebyshev() → Ultraspherical(1)

julia> push!([C2], C1)
2-element Vector{ApproxFunBase.ConcreteConversion{Chebyshev{ChebyshevInterval{Float64}, Float64}, Ultraspherical{Float64, ChebyshevInterval{Float64}, Float64}, Float64}}:
 ConcreteConversion : Chebyshev() → Ultraspherical(1.0)
 ConcreteConversion : Chebyshev() → Ultraspherical(1.0)

[codecov]

Codecov Report

Patch coverage: 69.23% and project coverage change: -9.40 ⚠️

Comparison is base (e3f9926) 71.68% compared to head (da0a6f8) 62.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #399      +/-   ##
==========================================
- Coverage   71.68%   62.29%   -9.40%     
==========================================
  Files          80       80              
  Lines        8456     8514      +58     
==========================================
- Hits         6062     5304     -758     
- Misses       2394     3210     +816     

Impacted Files	Coverage Δ
src/ApproxFunBase.jl	75.00% <ø> (-8.34%)	⬇️
src/Operators/banded/Reverse.jl	33.33% <0.00%> (-41.67%)	⬇️
src/Operators/general/PartialInverseOperator.jl	46.55% <0.00%> (-51.67%)	⬇️
src/Spaces/ArraySpace.jl	50.00% <0.00%> (-26.28%)	⬇️
src/Spaces/ContinuousSpace.jl	66.31% <0.00%> (-15.41%)	⬇️
src/Spaces/DiracSpace.jl	70.00% <0.00%> (-1.19%)	⬇️
src/Spaces/HeavisideSpace.jl	34.84% <0.00%> (-2.66%)	⬇️
src/Spaces/QuotientSpace.jl	38.20% <0.00%> (-43.99%)	⬇️
src/Spaces/SubSpace.jl	49.00% <0.00%> (-16.31%)	⬇️
src/Spaces/SumSpace.jl	67.33% <0.00%> (-3.98%)	⬇️
... and 24 more

... and 27 files with indirect coverage changes

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