Fix mesh partitioning

This MR fixes an issue when partitioning the mesh with a large number of processors. When removing boundary conditions and regions that do not appear on a given partition, the current logic is to remove the XML tag from the parent, and then request the sibling element of the tag. This, potentially, is the wrong order since the tag has already been removed from its parent. For certain choices of very large number of processes was causing a segmentation fault, which this MR fixes.