*By Evalf, and other Nutils contributors*

officialPoisson's equation

Solves Poisson's equation `Δu = 1`

subject to boundary constraints, using
the fact that the solution to the strong form minimizes the functional `∫ .5 ‖∇u‖² - u`

. The domain is a unit square, and the solution is constrained
to zero along the entire boundary.