[bugfix] fixed issue where raster to mesh conversion would fail if all all intensity values were negative
Merge branch 'master' of https://github.com/ufz/ogs
Empty GroundwaterFlowProcess and its Base classes.
[PL] Add ProcessLib to CMake files.
Add ProcessLib to doxygen inputs.
[PL] Add virtual initialize().
[PL] Add number of node variables.

Element variables are not yet used.
[PL] Add destructor for GroundwaterFlowProcess.
Adjusted GUI for removed FEMCondition classes
[PL] Include class Process into ProjectData.
[Tests] Removed TestBoostXmlCndInterface.

The test tested class BoostXmlCndInterface, that is removed previously.
[CLI] Vector for storing ProcessVariables.
Replace BaseLib::number2str with std::to_string.

Also remove the header StringTools.h everywhere.
Add a previously missing sstream include in DateTools.cpp.
Merge branch 'StdToString' into develop
[ProjectData] Make attribute name readable/consistent.
[ProjectData] Add configuration of project to ProjectData.
[ProjectData] Path argument for ProjectData constructor.
[ProjectData] Functionality to read geometry.
[ProjectData] Functionality to read mesh.
[ProjectData] Change deallocation order.

The processes first, because they depend on meshes; then the meshes.