add learning_rate.py

See the MAE-NT relation, how the model improves with more training data
update data for H2 and H4
do not commit change on *.pyc
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delete last line of energy data to match density data
Thomas gave me correct data
Merge branch 'delete-one-energy' into learning-rate
Thomas fix the miss density row in data
add rep in train_model() for k-fold cross validation
get learning rate data and figure
Merge branch 'learning-rate'