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.

Whether or not two meshes agree well numerically seems to depend on
how well connected two differently spaced mesh domains are.

This reverts commit 93f115a6.