Use BL_PROFILE instead of BL_PROFILE_VAR to time in knapsack()swap

[atmyers]

This timer was using BL_PROFILE_VAR without an associated BL_PROFILE_STOP, resulting in unused variable warnings. It seems to me that we can just use BL_PROFILE here and rely on scope to stop the timer.

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