Preserve ShapedAxis components during vcat

[dingraha]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Currently when calling vcat with two ComponentVectors, all multi-dimensional components of the second argument are flattened:

julia> x = ComponentVector(a=rand(Float64, 2, 3, 4), b=rand(Float64, 4, 3, 2))
julia> y = ComponentVector(c=rand(Float64, 3, 4, 2), d=rand(Float64, 3, 2, 4))
julia> xy = vcat(x, y)

julia> size(xy.a)
(2, 3, 4)

julia> size(xy.b)
(4, 3, 2)

julia> size(xy.c)
(24,)

julia> size(xy.d)
(24,)

julia> 

This PR fixes that.

[dingraha]

@ChrisRackauckas Could I get a review of this PR?

[ChrisRackauckas]

Sorry I had missed this one!

[dingraha]

@ChrisRackauckas No problem, and thank you!