Using float for time in the Parcels kernel

[erikvansebille]

This PR reverts to using float inside the kernel (as in v3), as performance tests have shown that this is approx 50% faster. See e.g. Parcels-code/parcels-benchmarks#1 (comment)

• Chose the correct base branch (main for v3 changes, v4-dev for v4 changes)
• Added tests

[erikvansebille]

Note that in this PR< we still assume that the user inputs particle.time, endtime, runtime, dt etc as np.datetime64 or np.timedelta64. This requires quite a bit of type-conversion under the hood. The code would het simpler if we expect (all/most of) these inputs to be floats too. Not sure if that's worth it, though

[VeckoTheGecko]

I think we need to adjust some things - not sure if that has to be in this PR though, happy for it to come in a different one

[erikvansebille]

We're not pursuing this anymore; getting good performance with numpy.datetime