Add new elastity demo (Python) using smoothed aggregation AMG.
Add pulley mesh to download list.
Add first rough (but working) version of c++ elasticity + AMG demo
Add C++ smoothed aggregation AMG demo.
Add missing UFL file for elasticity AMG demo
Add unit test for smoothed aggregation AMG.
Minor tweaks to new AMG demo.
Merge branch 'garth/fix-issue-430-samg' into next