Add FE and FFT tests that converge to same results

Both tests converge to the same energies with 1e-8 a.u. accuracy.
test_free_energy3: use spectral finite elements
Increase CG max iter to 800
solve_cg: precalculate the diagonal

This gives around 2x speedup.
Simplify precond()
openmp: add omp_get_wtime() interface
Use OMP in sparse matrix vector multiply
solve_cg() add timings