A link between a parameter and the data for that parameter.
The data for the parameter is available as in the preferred type and as
an Argument for the CLI.
<p>We also record validity information where applicable.
<p>For values, null and undefined have distinct definitions. null means
that a value has been provided, undefined means that it has not.
Thus, null is a valid default value, and common because it identifies an
parameter that is optional. undefined means there is no value from
the command line.
@constructor
function Assignment(param, requisition) {
    this.param = param;
    this.requisition = requisition;
    this.setValue(param.defaultValue);
}