Allow plotting only a subset of Amr levels

[maxpkatz]

Summary

If the user would like to limit plotfile size on disk, they can plot only a subset of the levels -- this should still be an accurate (though lower resolution) representation of the fluid state since the state is averaged down at the end of the timestep.

fcompare is also updated to allow comparing between plotfiles with different numbers of levels.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[WeiqunZhang]

I think we need to update this line too.

amrex/Src/Amr/AMReX_AmrLevel.cpp

Line 221 in 47347f7

int f_lev = parent->finestLevel();

[WeiqunZhang]

My suggestion is to revert the function interface change of Amr::writePlotFileDoit, because it's hard for AmrLevel to get the max_level_to_plot argument.

[maxpkatz]

I think we need to update this line too.

amrex/Src/Amr/AMReX_AmrLevel.cpp

Line 221 in 47347f7

int f_lev = parent->finestLevel();

Updated

My suggestion is to revert the function interface change of Amr::writePlotFileDoit, because it's hard for AmrLevel to get the max_level_to_plot argument.

I think the reason I did it this way is that writePlotFileDoIt doesn't know whether it's a small plot file or a full plot file. But I solved this problem for now by getting rid of small_plot_max_level, it seems unnecessary to have separate controls for both kinds of plotfiles for this particular feature.