Extrusion new

Support for extruded meshes is added for the SEQUENTIAL and OPENMP backends. 

ExtrudedSet extends Set to take the number of layers. 
ExtrudedMap extends Map to take the column offset used to directly address cells in the vertical direction. 

The generated code has the innermost loop over the column cells. 

The cache keys change from it_space.extents to it_space level.

A unit test for extrusion is added.

Three examples are added to the demo/ folder to exercise extruded meshes.  

In the OpenMP backend, the handling of global variables changes to avoid false sharing.