Two problems reading SBML

An error occurs when reading an SBML model in which parameter declarations are 
dispatched in the reactions definitions. While wainting for a code fix, it is
best to group parameter declarations in one section at the beginning. 

Next, GNU MCSim requires stoichiometries to be defined, and an error occurs if 
they are not. The code will be fixed (assuming value one when the stoichiometry 
is not given).