Added beta_Hs and beta_Hms to avoid confusion, match wiki

-Added parameters 17 (beta_Hs) and 18 (beta_Hms) to RNAi_Model
-Changed beta_H to beta_Hs and beta_Hms in Hs and Hms equations
-Also changed exponents of k_1 and k_2 from 6 to -3 (will likely change
these later)