Add string macro to define KnotVector{Int}

[hyrodium]

The following macro is not useful for most people but is useful for debugging and writing tests.

julia> using BasicBSpline

julia> macro knotvector_str(s)
           return sum(KnotVector(findall(==('0'+i), s))*i for i in 1:9)
       end
@knotvector_str (macro with 1 method)

julia> knotvector"1111"
KnotVector([1, 2, 3, 4])

julia> k1 = knotvector" 1 2  4 5"
KnotVector([2, 4, 4, 7, 7, 7, 7, 9, 9, 9, 9, 9])

julia> k2 = knotvector" 1 21 5 5"
KnotVector([2, 4, 4, 5, 7, 7, 7, 7, 7, 9, 9, 9, 9, 9])

julia> k3 = knotvector" 1 21 5 3"
KnotVector([2, 4, 4, 5, 7, 7, 7, 7, 7, 9, 9, 9])

julia> k1 ⊆ k2
true

julia> k1 ⊆ k3
false

[hyrodium]

I'm not sure we really need this macro in src, but I'm planning to add it just for fun.