Standardize solution matrix type

[adknudson]

AlternatingProjections returns a Matrix while others return a Symmetric.

G = rand(4, 4)
nearest_cor(G, Newton()) # returns LinearAlgebra.Symmetric{Float64}
nearest_cor(G, AlternatingProjections()) # returns Matrix{Float64}

The output matrix type should be consistent for all algorithms. This can be fixed by wrapping the solution in Symmetric in the build_ncm_solution method.