use abs for spherical 2d area

[zhichen3]

Summary

2d area in theta direction goes negative at theta=0 symmetry axis due to negative theta. But when we integrate from 0 to 2pi in phi, area orientation is the same regardless of +/- theta.

Additional background

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