Staggered mesh additions (small)

o Added CELL_CENTER as a synonym for CELL_CENTRE
  (US / British spelling)

o Added a test case to examples/test-staggered. Simulates
  waves in a periodic box, starting from a top-hat pulse.
  FFTs the result to calculate the dispersion relation,
  and compares the result between central and staggered schemes.

  To generate plot (test_staggered.pdf), run the Python script

  ./runtest

  in examples/test-staggered