How to use the nexusformat.nexus.NXprocess function in nexusformat

def save_fit(self):
        """Saves fit results to an NXentry"""
        self.read_parameters()
        group = NXprocess()
        group['data'] = self.data
        for f in self.functions:
            group[f.name] = self.get_model(f)
            parameters = NXparameters()
            for p in f.parameters:
                parameters[p.name] = NXfield(p.value, error=p.stderr, 
                                             initial_value=p.init_value,
                                             min=str(p.min), max=str(p.max))
            group[f.name].insert(parameters)
        if self.fit is not None:
            group['program'] = 'lmfit'
            group['version'] = lmfit.__version__
            group['title'] = 'Fit Results'
            group['fit'] = self.get_model()
            fit = NXparameters()
            fit.nfev = self.fit.result.nfev