Coarse-grid solve workspace

[andrewreisner]

The MSG workspace array is currently also being used as a workspace array for coarse-grid solve communication. When the redistributed solver is used, the coarse-grid global sizes should not be added to the allocation bound.

[andrewreisner]

Since the redistributed coarse-grid solver can replace the legacy kernels for the serial BoxMG coarse-grid solver, the coarse-grid workspace could be removed.