Added backward unit test for test_moving_eddies
Added backward unit test for test_moving_eddies

Added a test where particle are first run forward in time, and then
backward in time. If they return to same location, backward works well
(within error of RK4)

Note that code currently fails on JIT mode, because code fwd and bwd
are run with same compiled code

OceanPARCELS/parcels,OceanPARCELS/parcels