Bugfix PolyTEOS10_bsq kernel

[michaeldenes]

Updating the polyteos kernel to compute the seawater density (and not just the seawater density anomaly). This is related to Parcels-code/Parcels#2133

[VeckoTheGecko]

Looks good!

Would it be useful/easy to also add a unit test here? Such as in the v3 code?

from v3:

@pytest.mark.parametrize("mode", ["scipy", "jit"])
def test_TEOSdensity_kernels(mode):
    def generate_fieldset(xdim=2, ydim=2, zdim=2, tdim=1):
        lon = np.linspace(0.0, 10.0, xdim, dtype=np.float32)
        lat = np.linspace(0.0, 10.0, ydim, dtype=np.float32)
        depth = np.linspace(0, 2000, zdim, dtype=np.float32)
        time = np.zeros(tdim, dtype=np.float64)
        U = np.ones((tdim, zdim, ydim, xdim))
        V = np.ones((tdim, zdim, ydim, xdim))
        abs_salinity = 30 * np.ones((tdim, zdim, ydim, xdim))
        cons_temperature = 10 * np.ones((tdim, zdim, ydim, xdim))
        dimensions = {"lat": lat, "lon": lon, "depth": depth, "time": time}
        data = {
            "U": np.array(U, dtype=np.float32),
            "V": np.array(V, dtype=np.float32),
            "abs_salinity": np.array(abs_salinity, dtype=np.float32),
            "cons_temperature": np.array(cons_temperature, dtype=np.float32),
        }
        return (data, dimensions)

    data, dimensions = generate_fieldset()
    fieldset = FieldSet.from_data(data, dimensions)

    DensParticle = ptype[mode].add_variable("density", dtype=np.float32)

    pset = ParticleSet(fieldset, pclass=DensParticle, lon=5, lat=5, depth=1000)

    pset.execute(PolyTEOS10_bsq, runtime=1)

    assert np.allclose(pset[0].density, 1027.45140)