* this could cause problems when comparing coordinates

* should improve performance

* improves load balancing of computation of base node
* should be easier to vectorize by NEC compiler now

* this allows an interpolation method to define an interpolation stack to be complete and further computation is done by following methods

adds argument to routine yac_interp_weights_write_to_file which allows to specify how an already existing file is to be handled

* reads in cell centers from a scrip formated grid file and stores them as vertices of the returned basic grid

* when the local grid data of a process contained duplicated cells with differing global cells id, which are masked out by the cell core mask, an error was generated, because in the routine determine_dist_edge_ranks the core mask was not taken into account

* average with barycentric coordinates weightening could case an error
  for regular source cells, if one of the edges is on a pole

* HCSBB could cause an issue for regular cells that has two of its vertices at a pole. The triangulation would cae on of the two triangle to be a single line, which was not correctly handled.