write a PEST-style structure file

        Parameters
        ----------
        f : (str or file handle)
            file to write the GeoStruct information to

def to_struct_file(self, f):
        """ write a PEST-style structure file

        Parameters
        ----------
        f : (str or file handle)
            file to write the GeoStruct information to

        """
        if isinstance(f, str):
            f = open(f,'w')
        f.write("STRUCTURE {0}\n".format(self.name))
        f.write("  NUGGET {0}\n".format(self.nugget))
        f.write("  NUMVARIOGRAM {0}\n".format(len(self.variograms)))
        for v in self.variograms:
            f.write("  VARIOGRAM {0} {1}\n".format(v.name,v.contribution))
        f.write("  TRANSFORM {0}\n".format(self.transform))
        f.write("END STRUCTURE\n\n")
        for v in self.variograms:
            v.to_struct_file(f)