parameter object represents
a parameter, which is a value that typically remains constant during
a simulation. For example, you use parameters to define reaction rate
constants. In some circumstances it is useful to allow parameter values
to vary. In these cases you can specify a parameter as nonconstant.
When you create a parameter, you scope it to either a model or a reaction.
Parameters scoped to a model can be used (or referenced) by any expression (reaction, rule, or event) in the model.
Parameters scoped to a reaction can be used (or referenced) by only the reaction rate expression.