Benchmark problem updates

[A-CGray]

Purpose

Makes some small changes to the benchmark problems:

• Decreased the maximum allowable wing loading in case 3 from 650 to 600 kg/m^2
• Added a requirement that the SOB remains at the same spanwise location as the span changes
• Changed the structural sizing rules
• Specified a fixed stiffener pitch
• Added some benchmark analyses to be performed on the baseline wing
• Added a required polar to perform on optimised designs from cases 2 and 3

Also:

• Adds a document summarising the updated model and problems, contains some new required results.
• Adds a file containing an OpenMDAO group that can be used for all aircraft performance calculations
• Adds an example DVGeo setup using the provided FFD files