unbundle pugixml

[Nico Schlömer]

pugixml was introduced to DOLFIN in 2011 and greatly simplified the treatment of XML data (commit 5746fb0f6d969d8f6bf14405dd83933e3fb07b16). Since then, pugixml has become widely available [1,2,3,4], so perhaps it's time to unbundle it from dolfin/io.

[1] https://tracker.debian.org/pkg/pugixml

[2] https://packages.ubuntu.com/source/trusty/pugixml

[3] https://apps.fedoraproject.org/packages/pugixml

[4] http://brewformulas.org/Pugixml