Write `y_boundary_guards` to BOUT.dmp.*.nc

[mrhardman]

At present, metadata metrics and differences (dx, dy, etc) are written by the outputVars function

BOUT-dev/src/mesh/impls/bout/boutmesh.cxx

Line 3223 in cc8e29f

void BoutMesh::outputVars(Options& output_options) {

and

BOUT-dev/src/mesh/impls/bout/boutmesh.cxx

Line 3252 in cc8e29f

getCoordinates()->outputVars(output_options);

. The grid information written this way cannot be reused to start a new simulation because a consistent y_boundary_guards variable is not written to the same BOUT.dmp.*.nc files.

Including a consistent value for y_boundary_guards in BoutMesh::outputVars() would be a useful improvement to BOUT++.