Numbering reset for exercises for Physics text

For the Physics text a bug is present where the numbering for problems/exercises is not resetting after the Conceptual Questions section, but continuing to increment.  The fix for this is located in numbering-slots.less but this slot is OSC wide,  meaning if this slot is changed, we will incur regression testing for all texts.

Since this is a hot potato before we go to press for 2nd edition, a physics specific slot was created in physics.less for now to override the slot in numbering to avoid regression testing for expediency.  Once (@contexts) are working with refactoring we can target book specific fixes of this nature.