Improve draft paper on stiffness optimisation

[Woolfrey]

A few revisions are needed to improve the draft paper:

• Run experiments on ergoCub (see Test stiffness optimisation code on ergoCub #112 )
• Expand matrices/vectors for certain equations for clarity (e.g. stiffness $\delta\mathbf{w} = \mathbf{K}\delta\mathbf{x}$ can be expanded to show its 6DoF properties)
• Improve diagrams for human arms by using 3D models instead of 2D drawings

[Woolfrey]

Simulations of ergoCub have been used to show stiffness optimisation compared to a human:

There have been ongoing issues with the right wrist which have recently been resolved: robotology/icub-tech-support#1613

[Woolfrey]

Paper has been submitted to IEEE Transactions on Cybernetics for review.