add a model-dependent systematic term to the variance

Syntax: systematic [<modelName>] <model systematic error>

Set a systematic error term on the model to be added in quadrature to that on the data when evaluating chi-squared. The default value is zero.

If no <modelName> argument is given, then the <model systematic error> value becomes the default value for all current models and any models defined in the future. But if a <modelName> is specified, the value will only apply to that model and it will override the default systematic error setting.

The <modelName> argument can be specified only if a model with that name already exists. To set a value specifically for the default model (which has no name), <modelName> should be unnamed.