Add class to call specificPostTimeStep at t=0

[TaigoFr]

I think many of us suffer from not having our specificPostTimeStep processing also at t=0 (Weyl Extraction or any other extraction you do). This PR comes to fix it. I have done many hacks in the past to fix it, but it's quite tricky because specificPostTimeStep has to be called in reverse order of levels, so it cannot be really called in initialData or smth like that.

This was mostly done by @mirenradia, I just generalized it a bit and above all: try to make it really user friendly.

The idea is basically you can now add just a few lines to Main, and specificPostTimeStep will be called (at t=0). You just have to be careful with defining the first_step to m_time==0. instead of dt as we commonly had before (sometimes by default, so be careful) EXCEPT for the PunctureTracker, that is also called in Main at t=0, and hence a if (m_time != 0) guard saves it inside specificPostTimeStep.

But it really is quite general. You can call any GRAMRLevel function, including custom ones you may define in your own Example (hence why a template Level), and you may choose not to revert the order of the levels. Plus, it's also made to be an AMR Scheduler, mostly due to how I inherited @mirenradia's version, but it's nice that like that you can also choose to use it as a function that is called at regular time intervals (say every 10 timesteps). A bit overkill I agree, but who knows...

The name 'CallFunctionOverLevels' is not amazing, but seemed quite generic. Feel free to offer suggestions if you find better alternatives.

P.S. I also edited MareNostrum's Make.defs.local under to hood.

[KAClough]

A few minors changes suggested, I'll take it into my fork for testing.

[KAClough]

@TaigoFr if you rebase this branch to the master we could merge it next... I am happy to sign off.

One thing I'm a bit undecided about is the comment where you show what to do for scheduling:
// or call at post-plotLevel, at every 'some_interval'
etc

It is very useful info but it feels like this should either be actually used in another example (but I know they don't have a specificPostTimestep yet) or added to the wiki. Maybe you can add it to the wiki page:

https://github.com/GRChombo/GRChombo/wiki/Extraction-and-integration

And then once you update your PR #121 you could implement it there for the Kerr Example?

[TaigoFr]

Rebased.

I hope it's enough to update an example. I can update the Kerr Example of the ADMQuantities PR #121 after this is merged. We can leave a brief mention in the wiki if necessary.

[mirenradia]

I have a few very minor changes.

[mirenradia]

I am happy for this to be merged now.

[KAClough]

Me too!