Move executables to Applications folder

${CMAKE_CURRENT_BINARY_DIR}/../BaseLib is not necessary anymore.

tetgen as a submodule.

- moved ProjectData.* to Applications/ApplicationsLib
- removed Vrpn folder in DataExplorer

Unified mesh layer mapping

Related to #469.

This module is automatically checked out when OGS_BUILD_GUI=ON and the executable
gets packaged with the Data Explorer.

Tetgen source is hosted at https://github.com/ufz/tetgen.git

Disable warning C4996 on MSVC

added a base class to MeshLayerMapper and LayeredVolume for handling basic functionality and class structure

reworked prims-layer-generation to work from top to bottom and prefer older (lower) layers in cases of conflicting information

This warning appears in std::fill_n and can be ignored.

[PL] GroundwaterFlow.h -> GroundwaterFlowProcess.h.

Fixed wrong support point index at start and end point.

Skeleton project data

Update gtest to 1.7

When giving a custom comparison in std::sort use < as operator (not <=).

geometry->geometrical_set, patch->geometry.

This fixes crashes in debug mode (especially in some tests) in which case the
runtime checks this rule and quits execution on violation.