Add possibility to parse XML from string. Improve DDCond manual

[MarkusFrankATcernch]

BEGINRELEASENOTES

• Add possibility to parse XML from string

  • Please see Detector::fromXML() with in-memory string #288 for details.
  • The main interface was not changed. Rather a view to the Detector class supports this functionality.
  • The existing view dd4hep::DetectorLoad was enhanced to allow the required functionality:

   Detector detector = ....;
   /// So here is the actual test: We parse the raw XML string
   DetectorLoad loader(detector);
   loader.processXMLString(buffer,0);

  • An example was added (ClientTests/src/XML_InMemory.cpp, which illustrates the usage).

• Improve DDCond manual

  • Improve that the DDCond manual reflects the source code.
  • The possible plugins provided are not all described. This shall be future work.

• Enable the GDML reader plugin

  • Closes Add GDML import and export functionality; Move to python logging #275

ENDRELEASENOTES
Resolves #288