Docs: Add PyOP2 concepts
Docs: Add code examples to concepts
Concept docs: expand the section on data
Concept docs: expand par_loop section
More on par_loops, access descriptors, direct loop example
Add par_loop examples using Const and Global
Expand the introduction and the section on sets
Use sphinx shorthand to strip package prefix
Add reference markers for every (sub)section
Merge pull request #295 from OP2/doc/concepts

Add user documentation on PyOP2 concepts