getindex of range in IdentityMultiple

[schillic]

The type of ind has to be restricted to Integer and another method for StepRange (or the supertype OrdinalRange or even that one's supertype AbstractRange) needs to be added.

https://github.com/JuliaReach/MathematicalSystems.jl/blob/a456045dacbfb7f2bd7e57a7a2746d06c538b8a9/src/identity.jl#L87-L98

julia> I = IdentityMultiple(1.0, 2)
IdentityMultiple{Float64} of value 1.0 and order 2

julia> Diagonal(I)
ERROR: MethodError: no method matching isless(::Int64, ::StepRange{Int64,Int64})
Closest candidates are:
  isless(::Missing, ::Any) at missing.jl:66
  isless(::Real, ::AbstractFloat) at operators.jl:157
  isless(::Real, ::Real) at operators.jl:346
  ...
Stacktrace:
 [1] <(::Int64, ::StepRange{Int64,Int64}) at ./operators.jl:268
 [2] <=(::Int64, ::StepRange{Int64,Int64}) at ./operators.jl:317
 [3] getindex(::IdentityMultiple{Float64}, ::StepRange{Int64,Int64}) at .julia/dev/MathematicalSystems/src/identity.jl:93