Fix `maximum(abs, ::Fun)`

[jishnub]

Fix https://github.com/JuliaApproximation/ApproxFun.jl/issues/907

After this,

julia> x = Fun()
Fun(Chebyshev(), [0.0, 1.0])

julia> a = abs(x)
Fun(ContinuousSpace{Float64, Float64, PiecewiseSegment{Float64, Vector{Float64}}}(PiecewiseSegment{Float64, Vector{Float64}}([-1.0, -0.0, 1.0])), [1.0, 5.55112e-17, 1.0])

julia> using LinearAlgebra

julia> norm(x - a, Inf)
2.0

julia> norm(a - x, Inf)
2.0

[codecov[bot]]

Codecov Report

Patch coverage: 21.73% and project coverage change: -0.03% :warning:

Comparison is base (c31337a) 27.53% compared to head (74e44f0) 27.51%. Report is 9 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #570 +/- ## ========================================== - Coverage 27.53% 27.51% -0.03% ========================================== Files 80 80 Lines 8379 8382 +3 ========================================== - Hits 2307 2306 -1 - Misses 6072 6076 +4 ``` | [Files Changed](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/570?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/Operators/Operator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/570?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9PcGVyYXRvci5qbA==) | `51.98% <0.00%> (ø)` | | | [src/Operators/banded/CalculusOperator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/570?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9iYW5kZWQvQ2FsY3VsdXNPcGVyYXRvci5qbA==) | `17.21% <0.00%> (ø)` | | | [src/Spaces/SumSpace.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/570?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NwYWNlcy9TdW1TcGFjZS5qbA==) | `31.98% <0.00%> (ø)` | | | [src/specialfunctions.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/570?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3NwZWNpYWxmdW5jdGlvbnMuamw=) | `16.77% <0.00%> (ø)` | | | [src/Operators/general/InterlaceOperator.jl](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/570?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL09wZXJhdG9ycy9nZW5lcmFsL0ludGVybGFjZU9wZXJhdG9yLmps) | `23.56% <50.00%> (+0.09%)` | :arrow_up: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/JuliaApproximation/ApproxFunBase.jl/pull/570/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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