The authors would like to thank Robert J. Hanisch of the ST ScI for supplying part of the data used in this paper and for the support and computing facilities at that institute. This work was supported in part by the DGICYT Ministerio de Educación y Ciencia (Spain) under grant No. BP90-0478. Partial support was also obtained from the D. G. U. Generalitat de Catalunya. J. Llacer's work is supported by the U. S. Department of Energy under contract No. DE-AC03-76SF00098.