For all Poisson tests, we need to set coeffs1 (alpha) to 1.0 instead of 0.0.
Since rhs_f (rho_glob) was generated with the incorrect coeffs1 (alpha) in mind,
we will need to update the currently incorrect rhs_f (rho_glob) function.
--> Addressed in #64

Additionally the test RefinedGyroZoniShiftedCulham does not yield sufficiently good convergence rates to the exact solution.
The error doesnt improve with increasing node count.
Exact Weighted-Euclidean Error: 0.00020584
Exact Infinity Error: 0.00130423
When reaching this error, we don't see any further improvements, suggesting that there is a problem with this test case.

The other Culham test PolarR6GyroZoniShiftedCulham does work correctly though.
As a comparison here we get
Exact Weighted-Euclidean Error: 2.24275e-08
Exact Infinity Error: 9.00029e-08