Minor Release 0.6.0

Improvements in operator*() performance across the board. Essentially avoid 6x6 maths and work with 3D operators. Reduces memory 'traffic' and allows GLM optimisations to work, at the expense of some clarity.

Removed transpose accessors such as topLeftT and rightT. It is faster to use arb::transpose. Less interface clutter.

Removed unnecessary const declarations from method returns. This should facilitate compiler optimisation.