Constraints

[looooo]

To sew diagonals and vector-straps to the glider we need a new element type: constraints
This new constraints are handled as normal Elements (there influence is computed in the explicit_step function) But after the time integration of all nodes, they have to be called again to update their position. This will be done in a apply_constraints function.
Constraints are elements. Passing them as elements to the constructor of the case is possible, but I have no idea how to cast them to constraints. (Should be possible shared pointer cast)

TODO

• Delete constraint stuff
• Add class constraint based on element
• EdgeConstraint (the one we need)