demos for extensions added

extensions/base_problems/LinearElasticity.h

extensions/demo/CMakeLists.txt

extensions/demo/fsi

extensions/demo/init/fsi.dat.2d

extensions/demo/init/navierStokes_TaylorHood.inc.2d

extensions/demo/init/reinit.inc.2d

extensions/demo/macro/channel.2d

extensions/demo/macro/macro.square.2d

extensions/demo/macro/obstacle_in_channel.2d

extensions/demo/src/fsi_explicit/ElasticityNavierStokes.h

...nsions/demo/src/fsi_explicit/fluidStructureInteraction.cc

extensions/demo/src/fsi_explicit/navierStokes.h