Added empty finite element 2D lab.
Added section on triangular meshes to 2d FEM lab.
Labeling fix to finite element lab.
Added introduction to section on 2d FEA algorithm.
Described the definition of the stiffness matrix and the load vector for
the 2D Finite Element lab.
Merge pull request #664 from insertinterestingnamehere/finite_element

(WIP) 2D finite element