Quite serious problem in addition and multiplication of "double" numbers
in the non-table-based models: we didn't check if the resulting number
has the base inside the "allowed" interval [0, 9999], so we could get
"double" numbers with a base > 9999, which could cause problems.
[same update was done in the original_reaction_vs_reactant_oriented
branch, but here we must also apply it to VariablesModelOpaal, of
course]