Allow plotting only a subset of Amr levels

[maximumcats]

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
• [x] 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