* 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

* matplotlib.pyplot and netCDF4 can have issues with multithreading

* 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