How-to: Calculate reaction forces

Having a how-to on calculating reaction forces and comparing the accuracy would probably be nice.

Is a good first issue, so providing some details from Slack:

Calculating the reaction forces can be done in two ways in Ferrite,

1) Calculate the traction at the boundary using the solution field to get the correct strains (and thus stresses), and then integrate to get the total reaction on the boundary.
2) Get the residual vector on the boundary by assembling the internal force vector considering the solution field, and extracting the constrained degrees of freedom (I usually use a dummy constraint handler to do this).

The second option is more accurate (specifically it will give the exact result if the loading was a body load), but the first option is a bit easier to implement. Both are mathematically correct and will converge to eachother assuming a sufficiently fine mesh.