tion affinities (thermodynamic driving forces)

 For the inputs 'es_constraints' and 'es_options', see 'es_default_options'

 Fields in 'es_constraints' and 'es_options' relevant to this function:

   es_constraints.beta_M_fix     - predefined beta values (only used if non-zero)
   es_constraints.beta_A_fix     - predefined beta values (only used if non-zero)
   es_constraints.beta_I_fix     - predefined beta values (only used if non-zero)
   es_options.kinetic_law        - Rate law {'cs','ms', ...}
   es_o