BlockRange for AbstractArrays

[jishnub]

After this,

julia> r = rand(2,2)
2×2 Matrix{Float64}:
 0.681849  0.273193
 0.848369  0.335195

julia> r[BlockRange(r)]
2×2 Matrix{Float64}:
 0.681849  0.273193
 0.848369  0.335195

This also defines parent for a PseudoBlockArray to extract the wrapped array.

julia> P = PseudoBlockArray([1:6;], 1:3)
3-blocked 6-element PseudoBlockVector{Int64, Vector{Int64}, Tuple{BlockedUnitRange{ArrayLayouts.RangeCumsum{Int64, UnitRange{Int64}}}}}:
 1
 ─
 2
 3
 ─
 4
 5
 6

julia> parent(P)
6-element Vector{Int64}:
 1
 2
 3
 4
 5
 6

parent differs from Array, as this doesn't create a copy. This PR shouldn't be a breaking change, as P == parent(P) still holds, and the new definition matches the docstring of parent.