Kernel docs: introduction
Kernel docs: add kernel API
Kernel docs: add local iteration spaces
Kernel docs: add section on data layout
Kernel docs: Add example for flattened midpoint kernel
Kernel docs: simplify local iteration space example
Kernel docs: add (in)direct arg access diagrams
Kernel docs: include diagrams
Kernel docs: add iteration spaces diagram
Kernel docs: include iteration spaces diagram
Merge pull request #309 from OP2/doc/kernels

Add kernel API documentation