This is a simple test of Poisson gravity. It loads a sphere (problem = 1, 2) or
cube (problem = 3) of uniform density onto the grid. The goal is to determine
whether the calculated potential converges to the analytical potential
as resolution increases. Problems 1 and 2 are the same setup except for one
detail: in Problem 2, instead of using the density requested, we modify
the density on the grid so that the total amount of mass in the domain
is equal to the mass of a sphere of the requested diameter. Problem 1
uses the density requested by the user, and so it will not get the right
mass: the object will not be exactly spherical due to Cartesian grid effects.
