Add type parameter to Jacobi

[jishnub]

Using integer orders makes comparisons with integers easier, e.g. n + 0.5 is statically known to not be an integer if n is an integer.

On master

julia> @inferred ApproxFunBase.maxspace(Jacobi(1,1), Jacobi(2,2))
ERROR: return type Jacobi{ChebyshevInterval{Float64}, Float64} does not match inferred return type Any
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] top-level scope
   @ REPL[5]:1

After this PR, this is fully type-inferred.

julia> @inferred ApproxFunBase.maxspace(Jacobi(1,1), Jacobi(2,2))
Jacobi(2,2)

[codecov]

Codecov Report

Base: 60.58% // Head: 61.04% // Increases project coverage by +0.46% 🎉

Coverage data is based on head (1175fda) compared to base (0b7ecbb).
Patch coverage: 90.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
+ Coverage   60.58%   61.04%   +0.46%     
==========================================
  Files          28       28              
  Lines        2471     2490      +19     
==========================================
+ Hits         1497     1520      +23     
+ Misses        974      970       -4     

Impacted Files	Coverage Δ
src/Domains/Ray.jl	58.46% <75.00%> (+14.60%)	⬆️
src/Spaces/Jacobi/Jacobi.jl	63.10% <85.71%> (+0.73%)	⬆️
...c/Spaces/Ultraspherical/UltrasphericalOperators.jl	79.67% <87.50%> (-0.11%)	⬇️
src/Spaces/IntervalSpace.jl	75.00% <100.00%> (+8.33%)	⬆️
src/Spaces/Jacobi/JacobiOperators.jl	67.04% <100.00%> (+0.50%)	⬆️

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