Update/documentation

I have attempted to get most of the documentation up to date with this branch. The user documentation and distributed memory developer documentation have been updated with the latest features of the OP2 API and library. 

I have also added the airfoil documentation that was previously only made available on the OP2 website.

The developer doc is the one that is less up to date, given the extensive changes done to the CUDA back-end and code generator.

If every one can go through these documents and raise any issues, it would really help to continue the maintenance of OP2. This will be particularly important as we are looking forward to integrating OP2 into the production line at RR.