Attribution-NonCommercial 4.0 International. The version of record of this article, first published in Geometriae Dedicata, is available online at Publisher's website: http://dx.doi.org/10.1007/s10711-010-9500-0Excepto si se señala otra cosa, la licencia del ítem se describe como Attribution-NonCommercial 4.0 International. The version of record of this article, first published in Geometriae Dedicata, is available online at Publisher's website: http://dx.doi.org/10.1007/s10711-010-9500-0