Optimization tutorial does not update fan pressure ratio

[ngomezve]

The fan pressure ratio in the optimization tutorial (example_opt.jl) only gets updated as
ac.pare[iepif, ipclimbn, :] .= x[13] #Fan PR

However, the design fan pressure ratio is that at the start of cruise, ipcruise1. The above line means that the variable does not have an impact on the aircraft.