results.png: make KL.position and MOCK_POSITION power spectra lines
thicker so they are clearly identifiable.

loop.png: do not overwrite the intermediary loop plot, save one
for each iteration

Sample more often to make better use of the computation time.
Beforehand, when starting from zero, convergence was far from
reached in the time the demo ran, while much time was used
to optimize for the initial samples.

This reverts commit 7c125138.