New example, constructing global matrices

[ahojukka5]

@maikkirapo, could you write an example / tutorial how to construct global stiffness matrix and force vector using JuliaFEM. You can use Jupyter Notebook and follow instructions from here. Alternatively, write MarkDown file. After ready, do pull request. As a model, you can use this toy model. It's important to show explained code how to do this all. Remember to report what JuliaFEM and Julia version you are using.