Convert to float in transform/itransform

[jishnub]

With this, the following works:

julia> transform(Chebyshev(), 1:2)
2-element Vector{Float64}:
  1.5
 -0.7071067811865476

Fix JuliaApproximation/ApproxFun.jl#659

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (768625d) 75.07% compared to head (272ead2) 70.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #627      +/-   ##
==========================================
- Coverage   75.07%   70.50%   -4.58%     
==========================================
  Files          80       80              
  Lines        8439     8405      -34     
==========================================
- Hits         6336     5926     -410     
- Misses       2103     2479     +376     

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