[WIP] MLABecLaplacian spherical 2d support.

[zhichen3]

Summary

add support spherical 2d support for MLABecLaplacian.

Additional background

I'm trying to get add spherical 2d geometry support for MLABecLaplacian for MLMG since Castro uses the apply() function in MLMG to compute thermal diffusion source term. I think I mainly need to change applyMetricTerms() and unapplyMetricTerms().

I think the logic of the current code is to use applyMetricTerms() within applyMetricTermsCoeffs to modify the coefficients by the extra geometric term from divergence, i.e. $r^2$ for spherical in r-dir, and r for cylindrical in r-dir. Then unapplyMetricTerms is applied to the entire $L(\phi)$ after everything is calculated, which corresponds to the 1/r^2 and 1/r outside of the derivative in divergence.

My current logic is to modify the coefficient by $r^2 \sin(\theta)$ when dir=0 and $\sin(\theta)$ when dir=1, and when we unapply we divide by $\frac{1}{r^2\sin(\theta)}$ altogether, so that it is consistent with the divergence calculation.

I think I should also make sure compResidual(), getFluxes(), and getGradSolution() to work although it doesn't affect the apply() function.

getGradSolution() probably works since I modified compGrad().

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