FR: View sub-arrays as Complex with Real underlying array

It would be nice to be able to include complex variables/arrays in a ComponentArray that still behaves as an array of Reals. This is necessary when some other components are real, or for use in solvers that only take Real arrays.

This would look something like:

julia> arr = ComponentArray(a = 0, b = ComplexView([ 1+ 2im, 3 + 4im ]));

julia> collect(arr)
[ 0, 1, 2, 3, 4 ]

julia> arr.b[1]
1 + 2im

Start with ComponentArray and the proposed ComplexView behavior shown in the examples. Check how ComponentArray currently handles nested arrays and how Julia array views represent complex values. Done means supporting the demonstrated construction, collecting the underlying real values, and preserving complex indexing through arr.b.