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

renames routine yac_couple_config_set_output_filename to yac_couple_config_set_config_output_filename

Moritz Hanke authored 8 months ago and Nils-Arne Dreier committed 7 months ago

adds interface for yac_basic_grid and yac_basic_grid_data which computes the areas for all of its cells