Allow users to create custom `dvar` similar to `dlon`

[erikvansebille]

Now that we further cleaned up the Kernel loop in #2333, and are also outputting the last output written (a huge improvement with respect to v3), there is another major output-caveat left in the Kernelloop-tutorial:

### 2. Be careful with updating particle variables that do not depend on Fields.
While assigning the interpolated value of a Field to a Particle goes well in the loop above, this is not necessarily so for assigning other attributes. For example, a line like particle.age += particle.dt is executed directly so may result in the age being dt at time = 0 in the output file.

One way to fix this is to treat custom variables the same as lon, lat and z:

1. Also create a dvar for a custom variable var (note with to_write=False)
2. Add code in PositionUpdate() in kernel.py that result in something like:

particles.var += particles.dvar
particles.dvar = 0

This nicely aligns with the idea to frame Kernels more as ODE solvers (#2338), where the result of a call to RK2_integrator() would then be a dx.

Open question: should every custom Variable get a d-version, or only if users set a certain flag (and then what should be the default)? This is relevant since variables that won't change during the simulation don't need a d-version and fewer variables means less memory overhead.

[erikvansebille]

Actually, 2899e37 shows a unit tests where particle.age += particle.dt does work out of the box. SO it might be that, now with the new Kernel loop, the output caveat cited above is moot anyways. To be further explored...

[VeckoTheGecko]

Just catching up on notifications now.

I'm not sure I understand this issue - seems like this isn't a problem then? Not sure why this needs to be further explored.

Also - if there is an issue - I think it might be easier to reconsider where particle writing fits in the loop rather than adding additional d... variables

[erikvansebille]

Yes you’re right. I should have closed this after I found out that 2899e37 fixes the original issue