Allow N = 0 for Troe

[K20shores]

recode the Troe F_C exponent from 1 / (1 + (1/N) log^2) to N / (N + log^2), so that N = 0 is allowed. This will get us two of the 'custom' reactions in the RACM mechanisms, which are essentially Troe with only the Arrhenius k_0 and k_inf combo factor.