Updating stommel test case. RK4 works, RK45 doesn't
Updating stommel test case. RK4 works, RK45 doesn't

A rewrite of the Stommel test case, now with working RK4.
EE doesn’t work because assert tolerance is too low (EE is just not
good enough in this case)
RK45 doesn’t work at all (see Issue #88)

OceanPARCELS/parcels,OceanPARCELS/parcels