Increased resolution of spatial and angular grid.

So discretization errors don't make the plots look so ugly.
We don't use the planck_grid array right now, so delete it.
Fixed y-axis range to show all of J.

In commit 05341e4 ("Changed "value" of the Planck function from 1 to
5.") I increased the "value" of the Planck function, which in turn
increases the value of J, especially at depth. But I forgot to adjust
the plot axes accordingly. This corrects that mistake.
ACCELERATED LAMBDA ITERATION WORKS FOR REAL!

The convergence is extremely fast - about 2-3 iterations and it's
converged to machine precision.