Improve **postgresql_db_connection** function

[JulienLebranchu]

Add a new function to connect to a PG database

function (config) 
{
    postgresql_db_connection <- furdeb::postgresql_db_connection(
      user = config[['user']],
      password = config[['password']], 
      dbname = config[['dbname']], 
      host = config[['host']], port = config[['port']]))
    return(postgresql_db_connection)
}