Initial commit of periodic boundary problem in 3D.
Need to make it three d, it is currently the 2D driver code
3D linear elasticity problem now works
Added .gitignore file
modified:   periodic_load_3d.cc
Done the code for block diagonal preconditioning.
Have done 3D poisson and 3D linear elasticity. Needs to be cleaned up.

Modified the code to print out at the imposed inflow velocity.
I wanted to confirm if it was correct or not.
modified:   lagrange_annular_wedge/two_d_annular_wedge.cc:

modified the label.
modified:   lagrange_square/sq_lgr.cc

Put in break preconditionable elements,
modified:   poisson_three_d/poisson_3d.cc