* takes properties into account, that were generated by the recent rework of dist_grid
* reduces complexity of the code

Replace one Netcdf_writer test with plotter. NetCDF4 seems to be not thread-safe.

makes sure that test_interp_method_avg_parallel produces consistent results independent of compiler optimisation

* in case a source cell is not covered by any target cell, its centroid
  is now computed based on its vertices