Allow plotfiles to include only a subset of the levels

[TaigoFr]

@KAClough and I discussed it would be good to allow the user to output only a subset of the levels (e.g. only the finest level) in plotfiles. This would allow the user to output more variables in plotfiles while still saving space.

The main motivation came from the AHFinder. Postprocessing the AHFinder needs a lot of variables (chi, hij, K, Aij), but outputting all of them to plotfiles is costly, when in reality only the finest level is needed (where the AH is). For example, for binaries, one could export only the finest level AND one coarse level (say, level 2) such that one can both do AHFinder postprocessing AND GW postprocessing (on the coarser level).

[mirenradia]

This sounds look a good idea.

[KAClough]

This would also allow us to do outputs at intermediate timesteps and so have better time resolution.

@mkunesch might have already included some of the tools to do this here:
https://github.com/GRChombo/Experimental_tools/tree/master/IntermediateCheckpointing

It should be straightforward to adapt to just output a single level though - currently Chombo just cycles through them and outputs the data for each in turn.

[mirenradia]

I just tried this. It's virtually trivial to implement but VisIt (at least v.3.1.2) can no longer open the files. It complains that the file Does not contain all refinement levels.

[mirenradia]

In any case, I have pushed the code to the enhancement/select_plot_levels branch if you want to try it out.