Add category and author nodes

add_category_and_author_nodes.py

categories = pd.DataFrame(df[['category_list']])
categories.rename(columns={'category_list':'category'},
                  inplace=True)
categories = categories.explode('category') \
                       .drop_duplicates(subset=['category'])

authors = pd.DataFrame(df[['cleaned_authors_list']])
authors.rename(columns={'cleaned_authors_list':'author'},
               inplace=True)
authors=authors.explode('author').drop_duplicates(subset=['author'])

add_categories(categories)
add_authors(authors)
add_papers(df)