For implicit, set ng_fieldgather = ng_alloc_EB

[dpgrote]

For the implicit solver, the values in the guard cells need to be consistent for the various grid operations.

[JustinRayAngus]

What do you think about combining this block with lines 325-331 and move them to after line 389? It would keep the implicit alterations to fewer blocks.

if (evolve_scheme == EvolveScheme::ThetaImplicitEM ||
    evolve_scheme == EvolveScheme::SemiImplicitEM ||
    evolve_scheme == EvolveScheme::StrangImplicitSpectralEM) {
    // For these implicit schemes, the number of ghost cells
    // for EB gather must be consistent with those for J.
    ng_alloc_EB.max( ng_alloc_J );
    ng_FieldGather.max( ng_alloc_J );
}