Outside is not so important, as long as >100 points. Inside close to r=0 also not sensitive, but getting closer (> 75% * R) to boundary needs better mesh. Presumably due to high initial gradient.

Code can be run for nu=10^-15 or 10^-6, significant deviations for precision=1.

Also introduce check for mass conservation in prob_laplace. Currently
does not sum to 1, a factor of 2 is missing somewhere.

This is done with a loop that iteratively creates array 'x' by starting
at zero and continuing until gradient is hit, at which point the mesh
starts becoming finer and then coarser again.