Add a new on-disk preset and write it to the configured
            presets path.

            :param preset: the new PresetDefaults to add

def add_preset(self, name=None, desc=None, note=None, opts=SoSOptions()):
        """Add a new on-disk preset and write it to the configured
            presets path.

            :param preset: the new PresetDefaults to add
        """
        presets_path = self.presets_path

        if not name:
            raise ValueError("Preset name cannot be empty")

        if name in self.presets.keys():
            raise ValueError("A preset with name '%s' already exists" % name)

        preset = PresetDefaults(name=name, desc=desc, note=note, opts=opts)
        preset.builtin = False
        self.presets[preset.name] = preset
        preset.write(presets_path)