Made periodic_load_3d to work with new headers.
renamed:    linear_elasticity_3d/periodic_load_3d.cc -> linear_elasticity_3d/lin_elas_qele.cc
New files for linear elasticity problem.

Changes to be committed:
modified:   linear_elasticity_3d/.gitignore
modified:   linear_elasticity_3d/Makefile.am
new file:   linear_elasticity_3d/lin_elas_lele.cc