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A modified version of this paper is under process and with a new title and abstract. Hence, this version of the article is completely withdrawn.

The original version of this paper has been withdrawn by the authors and merged with the revised version of astro-ph/9702014.

This paper has been withdrawn by the authors. Significantly revised versions of the results of this paper are now available in arXiv:0707.0487v2 and arXiv:0808.3169v1.

This paper is withdrawn.

This paper has been withdrawn.

This paper has been withdrawn. (Reason) Its contents have been entirely superseded by the contents of the articles arXiv:0809.3444 and arXiv:0705.3070. There is no profitable reason to keep it alive. No material on it is however wrong.

This paper has been withdrawn by the author. A revised and expanded version is gr-qc/9907028 (Phys.Rev. D60 (1999) 104043).

This paper is withdrawn because its results have been previously reported in arxiv hep-th/0507200.

This paper has been withdrawn

This paper has been withdrawn by the author, due to an error in Proposition 2.2.

This paper has been withdrawn because the content has been substantially improved in a later paper, arXiv:0806.1165.

This paper has been withdrawn by the authors.

This paper has been withdrawn by the authors due to a likely hole in the proof.

This paper has been withdrawn by the author due to an error

This paper has been withdrawn.

This paper has been withdrawn by the author because has been send to other journal.

This paper has been withdrawn by the author because overcame by arXiv:0910.4694

This paper has been withdrawn.

This paper has been withdrawn by the author, due to necessity of revision.

This paper has been withdrawn by the author. The most updated version can be accessed by arXiv:1806.07290.