Add option to fill ghost cells in average_cellcenter_to_face

[BenDavisonPetch]

Summary

Adds an optional parameter ngrow to average_cellcenter_to_face and two new overloads that take an array of AMREX_SPACEDIM IntVects. average_cellcenter_to_face fills this many ghost cells in the face-centred MultiFabs.

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

[WeiqunZhang]

Also, let's not worry about the versions using Vector<MultiFab*>. We have that for backward compatibility, otherwise we would have removed them. The Array version is better because it has explicit size.

[BenDavisonPetch]

Also, let's not worry about the versions using Vector<MultiFab*>. We have that for backward compatibility, otherwise we would have removed them. The Array version is better because it has explicit size.

Makes sense; I've removed the new Vector<MultiFab*> overload.