Add copy(::SubKnotVector)

[hyrodium]

Before this PR

julia> using BasicBSpline

julia> k = KnotVector(1:8)
KnotVector([1, 2, 3, 4, 5, 6, 7, 8])

julia> view(k, 1:3)
SubKnotVector([1, 2, 3])

julia> copy(view(k, 1:3))
ERROR: MethodError: no method matching copy(::SubKnotVector{Int64, SubArray{Int64, 1, Vector{Int64}, Tuple{UnitRange{Int64}}, true}})

Closest candidates are:
  copy(::Base.ReshapedArray{<:Any, 2, <:SparseArrays.AbstractSparseMatrixCSC})
   @ SparseArrays ~/.julia/juliaup/julia-1.9.3+0.x64.linux.gnu/share/julia/stdlib/v1.9/SparseArrays/src/sparsematrix.jl:508
  copy(::Base.IdSet)
   @ Base idset.jl:16
  copy(::LinearAlgebra.Tridiagonal)
   @ LinearAlgebra ~/.julia/juliaup/julia-1.9.3+0.x64.linux.gnu/share/julia/stdlib/v1.9/LinearAlgebra/src/tridiag.jl:613
  ...

After this PR

julia> using BasicBSpline

julia> k = KnotVector(1:8)
KnotVector([1, 2, 3, 4, 5, 6, 7, 8])

julia> view(k, 1:3)
SubKnotVector([1, 2, 3])

julia> copy(view(k, 1:3))
KnotVector([1, 2, 3])

[codecov]

Codecov Report

Merging #341 (c7918bf) into main (4259f0c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #341   +/-   ##
=======================================
  Coverage   97.02%   97.02%           
=======================================
  Files          14       14           
  Lines        1546     1547    +1     
=======================================
+ Hits         1500     1501    +1     
  Misses         46       46           

Files	Coverage Δ
src/_KnotVector.jl	95.45% <100.00%> (+0.02%)	⬆️