Improved branch identification

This version includes improvements in branch identification algorithm used by second and third stage. The n_dir array technique is completely replaced by the more versatile mdrid_n_xy one (array of array-coordinates) in all the three stages.

It is tested on the complete SRTM N39E021, just like 2021-04. See GitHub action 3943311051. The performance is improved mostly because of the optimizations in arrange_lines, but also by the slow assert-s made optional.

Summary of the major changes:

• New accumulate_by_mgrid function, that allows dropping of n_dir technique and neighbor point limitations
• Optimization in arrange_lines to use sorted pend_lines array
• Final geometry generation improvements, like optional geometry layers, line smoothing and OSM tags

Work in progress:

• Support for full cylinder/sphere wrapped around maps (allowed after the transition to mdrid_n_xy)
• More optimizations by accumulating the coverage area of each point in parallel
• Experiments with flattened mgrid_n_xy array (converted to array of indices)