First commit to new extrusion branch. Previous functionality ported.
Fixed global attribute error when backend changes.
Extra flags added to OpenMP.
IterationSpace gets a cache_key property using only extents and layers
Fixed unit test errors for sequential and OpenMP backends.
Fix OpenMP globals. OpenMP and sequential are compatible with trunk.
Extrusion test added.
Computeind moved to pyop2 folder and added to ext.
Code refactoring.
Fixed padding to take into account different data types.
Eliminated redundant code from computeind.pyx.
Create a helper function to add the offsets to the list of args.
Offset deleted from Map. Extruded map default dim set to 1 and layers to 2.
Eliminated the triangle_reader file dedicated to the extrusion case.
Tidying up code.
Eliminate ExtrudedSet and ExtrudedMap types. Use optional args.
Change layers in IterationSpace.
Set default partition size in extrusion demos
Merge pull request #182 from OP2/extrusion-new

Adding extruded mesh capabilities to PyOP2 for the Sequential and OpenMP backends.
Set and Map objects have been modified to accept extrusion-specific parameters.
Two examples have been added to the demo folder to exercise extrusion.
Relevant unit tests have been added.