solve poisson in arbitrary dimensions

[luiswirth]

I should convert the poisson{1d,2d} examples into a poisson example that is generic over the dimension.

[luiswirth]

In commit 3f2549bdf2d6abc6b4dd4d2fb763c44578704f73 this was done. Unfortunatly in dimensions higher than 1D the convergence rate is terrible. Not clear why.

[luiswirth]

Here we can see that we converge to the wrong solution. It looks like at x=1 a wrong boundary condition is being enforced.

[luiswirth]

As suspected there was a bug in the boundary condition enforcement. It is now fixed.

The 2D solution now correctly converges with order 2 in L2 norm as predicted by theory. 🥳

[luiswirth]

convergence in 3D is however still terrible... Unclear why.

----------------------------------------------
|  k | nsubdivisions |     error | conv_rate |
----------------------------------------------
|  0 |             1 |   0.000e0 |       inf |
|  1 |             2 |  4.078e-6 |      -inf |
|  2 |             4 |  3.781e-1 |    -16.50 |
|  3 |             8 |  3.231e-1 |      0.23 |
|  4 |            16 |  2.966e-1 |      0.12 |
|  5 |            32 |  2.776e-1 |      0.10 |
----------------------------------------------

[luiswirth]

Beginning at 3 subdivisions the solution in 3D is completly spurious.

[luiswirth]

closed in favor of #28