Anisotropic exchange interactions in the copper(II) and vanadium(IV) dimers [(L')Cu(.mu.-OH)2Cu(L')](ClO4)2 and [(L)VO(.mu.-OH)2VO(L)]Br2 with 1,4,7-triazacyclononane (L) and its N,N',N"-trimethyl derivative (L'): a single-crystal EPR study