Added candidate for systems_of_equations_ex7
Removed det(F) from source term, since it shouldn't be there according to formulation from Ciarlet.
Removed global pointer, it's not needed
Added linesearch option to run.sh
Improved introduction comments
Added stress calculation to the example
Fixed unused variable warnings
Added systems_of_equations_ex7 to examples/Makefile.in and examples/Makefile.am
Use a fixed-length arrays, avoid heap allocation.

Also reduce the default number of elements so that it runs fast enough in DBG mode.
Merge branch 'systems_of_equations_ex7'