Calculation of annual_electrical_output in costs_2015.py

In `costs_2015.py` the calculation for the annual electrical output is as follows:

`self.annual_electric_output = self.mean_electric_output * 24.0e0 * 265.25e0`

Should the 265.25 not be 365.25 to represent a full year or is there a calculated 100 days of downtime for maintenance?
