When attributes are missing the corresponding element is outputted in
a JSON representation. Therefore I added
BaseLib::propertyTreeToString(boost::property_tree::ptree const& tree)

Fixes #590.

Also remove the header StringTools.h everywhere.
Add a previously missing sstream include in DateTools.cpp.

Place it in StringTools.

moved functions getFileNameFromPath(), getSuffixFromPath(), copyPathToFileName() and extractPath() from files StringTools.{cpp,h} to files FileTools.{cpp,h}

moved functions trim(), replaceString(), splitString() and template functions number2str() and str2number() into namespace BaseLib

added recent changes concerning the attachment of time series sensor data to station objects from ogs5 to ogs6, added RapidXml as a small library for reading XML files from console application, added existing xml schema files

in order to make the name of libraries, namespaces and directories more consistent the Base directory is know named BaseLib

- added test programm for solving a linear system with CG-method
- time measurement is at the moment OS dependent!
- added Base folder
- added GeoLib folder
- added files in MathLib folder - restructured MathLib folder