feat: `ngen_cal_model_configure` model hook

[aaraney]

Add model hook: ngen_cal_model_configure hook that mirrors the existing ngen_cal_configure hook.

    @hookspec
    def ngen_cal_model_configure(self, config: ngen.cal.model.ModelExec) -> None:
        """
        Called before calibration begins.
        This allow plugins to perform initial configuration.

        Plugins' configuration data should be provided using the
        `plugins_settings` field in the `model` section of an `ngen.cal`
        configuration file.
        By convention, the name of the plugin should be used as top level key in
        the `plugin_settings` dictionary.
        """

Additions

• ngen_cal_model_configure model hook; Called before calibration begins. This allow plugins to perform initial configuration.
• unwrap method added to ngen.cal.ngen.Ngen. Returns the instance's __root__ attribute.

Changes

• Agent now takes a Model instance as a parameter. Previously, the raw config was passed.