#2219 Remove N <: Real for zonotopic sets

[mforets]

see #2219

[mforets]

Got exception outside of a @test
  LoadError: MethodError: linear_map(::Array{Float64,2}, ::BallInf{Float64,Array{Float64,1}}) is ambiguous. Candidates:
    linear_map(M::AbstractArray{N,2}, P::AbstractPolyhedron{N}; algorithm, check_invertibility, cond_tol, inverse, backend, elimination_method) where N<:Real in LazySets at /home/travis/build/JuliaReach/LazySets.jl/src/Interfaces/AbstractPolyhedron_functions.jl:593
    linear_map(M::AbstractArray{N,2}, Z::AbstractZonotope{N}) where N in LazySets at /home/travis/build/JuliaReach/LazySets.jl/src/Interfaces/AbstractZonotope.jl:340
    linear_map(M::AbstractArray{NM,2}, P::AbstractPolyhedron{NP}; kwargs...) where {NM<:Real, NP<:Real} in LazySets at /home/travis/build/JuliaReach/LazySets.jl/src/Interfaces/AbstractPolyhedron_functions.jl:656
  Possible fix, define
    linear_map(::AbstractArray{N,2}, ::AbstractZonotope{N}) where N<:Real
  Stacktrace:
   [1] affine_map(::Array{Float64,2}, ::BallInf{Float64,Array{Float64,1}}, ::Array{Float64,1}) at /home/travis/build/JuliaReach/LazySets.jl/src/Interfaces/LazySet.jl:405
   [2] top-level scope at /home/travis/build/JuliaReach/LazySets.jl/test/unit_BallInf.jl:135
   [3] include(::String) at ./client.jl:439
   [4] macro expansion at /home/travis/build/JuliaReach/LazySets.jl/test/runtests.jl:84 [inlined]
   [5] macro expansion at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.4/Test/src/Test.jl:1113 [inlined]
   [6] macro expansion at /home/travis/build/JuliaReach/LazySets.jl/test/runtests.jl:84 [inlined]
   [7] macro expansion at ./util.jl:175 [inlined]
   [8] top-level scope at /home/travis/build/JuliaReach/LazySets.jl/test/runtests.jl:84
   [9] include(::String) at ./client.jl:439
   [10] top-level scope at none:6
   [11] eval(::Module, ::Any) at ./boot.jl:331
   [12] exec_options(::Base.JLOptions) at ./client.jl:264
   [13] _start() at ./client.jl:484
  in expression starting at /home/travis/build/JuliaReach/LazySets.jl/test/unit_BallInf.jl:1