Write information_gain to result file

bilby/core/result.py

@@ -577,7 +577,7 @@ class Result(object):
             'log_noise_evidence', 'log_bayes_factor', 'priors', 'posterior',
             'injection_parameters', 'meta_data', 'search_parameter_keys',
             'fixed_parameter_keys', 'constraint_parameter_keys',
-            'sampling_time', 'sampler_kwargs', 'use_ratio',
+            'sampling_time', 'sampler_kwargs', 'use_ratio', 'information_gain',
             'log_likelihood_evaluations', 'log_prior_evaluations', 'samples',
             'nested_samples', 'walkers', 'nburn', 'parameter_labels',
             'parameter_labels_with_unit', 'version']