Add automatically assigned detail levels to each node

in a way that is remotely similar to Douglas-Peuckert.
The detail level gets assigned via
GeoDataLineStrings::optimized() which returns another
lineString that holds detail level information.
This call is currently made inside the PN2 and
OSM file parser.
The algorithm works best for long linestrings.
Therefore the maximum benefit is made e.g. for
the atlas map theme where performance increases
by about 10-20%.