Magnetic exchange interactions in some novel .mu.-azido-bridged copper(II) dimers. Crystal structures of [L2Cu2(.mu.-N3)(N3)2](ClO4).cntdot.H2O, LCu(N3)2, [L2Cu2(.mu.-N3)2(ClO4)2], and L'Cu(N3)2 (L = N,N',N"-trimethyl-1,4,7-triazacyclononane and L' = 1,4,7-triazacyclononane)