Combined Density Functional Theory and Molecular Mechanics (QM/MM) Study of Single-Site Ethylene Polymerization and Chain Termination for the Catalysts [(C~6R~5N=CH)C~4H~3N]~2PrTi^- (R = F, H) and [bis(eta^5-1-indenyl)dimethylsilane]PrZr^+ in the Presence of the Counterion CH~3B(C~6F~5)~3^-