Updated implementation for European catchments

• We updated the functions in libs
• We updated the jupyter notebook "interpret_lstm_eu.ipynb"

New implementation for European catchments

• We added a new jupyter notebook "interpret_lstm_eu.ipynb" to show the use of integrated gradients to interpret LSTM-based hydrological models for European catchments.
• We made some modifications to the readme file.
• We updated some information in "interpret_lstm_us.ipynb".

First release after accepting

• We added missing codes.
• We made some modifications to the readme file.

Release before accepting

• We added a jupyter notebook to show how to use the expected gradients and additive decomposition methods to interpret the LSTM-based hydrological model.
• We reorganized the directory.
• We made some modifications to the readme file.

First release before accepting

This is the first release before accepting.