Changed the output_weight for the event based calculator

An event based calculation with 10,000 SES x 1 year generates the same output of a calculation of 1 SES x 10,000 years: in other words, the output_weight must be proportional to the product  `ses_per_logic_tree_path * investigation_time`.
Please add a comment that explain better the mean of output_weight and what feature achieve.