DataHandler with multiple stats per variable

Implement new strategy to interoperate one variable with multiple statistics. For example: stats_per_var = {'o3': ['dma8eu', 'perc95', 'perc05'], 'relhum': 'average_values'}

• upload downloads
• reset names with statistic name (e.g. o3_dma8eu, o3_perc95, o3_perc05)
• update variable list in DataHander with updated names
• implement statistics selector for target variable