cpp: bioformats: Add escapeXML and sanitizeXML to XMLtools
cpp: xerces: Add writeDocument methods
cpp: bioformats: Replace C++11 pop_back for std::string
cpp: xerces: Work around lack of path addition in older Boost.Filesystem
cpp: xerces: Generalise document writing to all nodes
Merge pull request #1478 from rleigh-dundee/cpp-xmltools

C++: Add helpers for XML serialisation