Field Sampling fails when particleset has no time set yet

[erikvansebille]

Parcels version

v4

Description

While #2622 implemented sampling of a Field directly with a particleset (var = fieldset.field[pset]) I now realise that this does not work if the particle times are not set explicitly.

In that case, (because time is set to NaN) the interpolation returns zero - without warning!

Code sample

fieldset.U.data[:] = 2.
pset = ParticleSet(fieldset, x=0., y=0.)
pset.sample, _ = fieldset.UV[pset]  # Sample initial value
np.testing.assert_allclose(pset.sample, 2.0, rtol=1e-12)

AssertionError: 
E       Not equal to tolerance rtol=1e-12, atol=0
E       
E       Mismatched elements: 1 / 1 (100%)
E       Max absolute difference among violations: 2.
E       Max relative difference among violations: 1.
E        ACTUAL: array(0)
E        DESIRED: array(2.)

Note that the test above passes if we change

- pset = ParticleSet(fieldset, x=0., y=0.)
+ pset = ParticleSet(fieldset, x=0., y=0., t=np.timedelta64(0, "s")