・Made slight modifcations to Remark 1.5.4, (iii) ・Made slight modifcations to Proposition 1.6, (iii) ・Made slight modifcations to Corollary 1.10, (a), (f) ・Added Remark 1.10.4 ・Made slight modifcations to Proposition 3.2, (iii) ・Updated the bibliography