renames routines yac_compute_concave_overlap_areas and yac_compute_concave_overlap_info to yac_compute_overlap_areas and yac_compute_overlap_info respectively

* user_callback.func_compute_weights_key is a char[] and therefore never NULL

* a compiler had issues with trigonometric function for -O0

Sergey Kosukhin authored 2 years ago and Moritz Hanke committed 2 years ago

* Note that calling YAXT functions in the tests requires additional
  modifications: ACX_SHLIB_PATH_VAR should be expanded before the checks so we
  could set LD_LIBRARY_PATH (or its alternative) to tell the dynamic linker
  where the YAXT library is.

improves support for concave target cells in routine yac_compute_concave_overlap_info and yac_compute_concave_overlap

* centre point of target polygon is not needed anymore