Add function to transfer data between callbacks and matrices

[frapac]

Before that PR, the data transfer occurred directly in the main function, preventing Julia compiler to infer type properly. This resulted in additional --- and unwanted --- allocations. A quick profiling using BenchmarkTools gives on main:

julia> include("profiling/qptest.jl")
  520.067 μs (6005 allocations: 141.88 KiB)
:Solve_Succeeded

By moving the transfer routine in a separate function, we allow a better specialization in the compiler, and a decrease in a number of allocations as types are inferred properly. With this PR, we get:

julia> include("profiling/qptest.jl")
  177.643 μs (49 allocations: 33.17 KiB)
:Solve_Succeeded

See #10

[youngdae]

Interesting. It seems that modularization greatly helps optimize the code.