Parallelise islands boundary nodes read

[brey]

For high resolution global coastlines this becomes a bottleneck due to the very large number of island geometries:

pyPoseidon/pyposeidon/mgmsh.py

Lines 110 to 117 in a3bff4b

	for row in ibs.itertuples(index=True, name="Pandas"):
	inodes, xyz = gmsh.model.mesh.getNodesForPhysicalGroup(dim=getattr(row, "dim"), tag=getattr(row, "tag"))
	db = pd.DataFrame({"node": inodes - 1})
	db["type"] = "island"
	db["id"] = -(getattr(row, "Index") + 1)
	bounds.append(db)