ptions.h                  - vector of Hill exponents (one for each reaction)
   es_options.no_equilibrium     - flag: assume no equilibrium for inactive reactions
   es_options.KV_prior_mean      - .. and KV value to be used in this case
   es_options.flag_test          - Run tests? (Boolean)
   es_options.flag_second_order  - Compute second-order elasticities? (Boolean)
   es_options.zv                 - Flux weights for 2nd order resp. coeff output function
   es_options.zc                 - Conc. weights