Roundoff domain bugfixes

[atmyers]

This fixes two bugs. We now:

1. Always recompute offset, dx, and inv_dx whenever the roundoff domain is computed, to make sure it is up-to-date.
2. Recompute the roundoff domain when Domain() is changed, as well as ProbDomain().

MFIX triggered these because it sometimes grows the ProbDomain(), and only later changes the Domain().

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
• are described in the proposed changes to the AMReX documentation, if appropriate