Load networks from SQL database

[hackl]

It would be great to have a more streamlined and user-friendly method for loading networks directly from an SQL database. Currently, the process works using the following code:

conn = sqlite3.connect("networks.db")
df_lotr = pd.read_sql_query('SELECT DISTINCT source, target from lotr', conn).rename(columns={"source": "v", "target": "w"})
conn.close()
n_lotr = pp.io.pandas.df_to_graph(df_lotr).to_undirected()

[M-Lampert]

I've submitted a PR (#303) implementing this feature. I've implemented the functions in line with the suggestions in #301 for the entire io module.