Fix/paralllel xml file

Fix issues when not all (or at least the root) partition of a parallel xml fld file is not present. After this fix this will now process the available partitions.

For example this fixes errors in wss (--nparts) module if some parts don't touch boundary.

There is also a small fix to FieldUtils::OutputInfo.cpp which now checks the partition file exists before trying to read it.

Finally there is a small fix to Octree.cpp which has a small bug when compiling on clang and is only currently visible on a mac since CI is not covering this currently.