GAMG processor agglomeration broken

Summary

GAMG with processor agglomeration is broken in 3852f7c5

Steps to reproduce

Run tutorial basic/laplacianFoam/implicitAMI-nonblocking

What is the current bug behaviour?

sigsegv

Environment information

• OpenFOAM version : 3852f7c5

Possible fixes

• processor agglomeration uses in-place gather
• so does not allocate space on slave ranks
• which at next level leads to zero sized coarse levels