Add save_and_reload_df decorator and minor modifs

[louismartin]

Add save_and_reload_df decorator and minor modifs

Usage:

@save_and_reload_df
def my_func():
    # Create and fill the dataframe
    df = pd.DataFrame(...)
    df = ...
    return df

What the decorator does: On the first execution of the function, it will retrieve the returned dataframe and save it at data/my_func.csv. When the function is called again, if the file data/my_func.csv exists, it will not call my_func() but just read the file and return the dataframe. If one wants to compute the dataframe again, just call my_func(overwrite=True) (argument added automatically by the decorator), the decorator will call the function, save and return the new dataframe.