Add a fwd include file, and a explicit template
instantiation definition in own compilation unit.

This decouples ogs.cpp from GWF process.

renamed struct for type of element from FEMElemType to CellType and renamed virtual method getFEMType() to getCellType() in Elemnts class hierarchy

added (1) FEMElemType to elements as template parameter, (2) method getFEMType() and (3) an enum for FEM element types

renamed Tri.h to TemplateTri.h and separated class template implementation of class template TemplateTri to TemplateTri.hpp

added functionality to find all nodes that are connected via edges and via element to other nodes; replaces all cout's with appropriate logog messages

fixed gmshinterface; fixed mesh copy constructor; fixed layer mapping; removed various TODO6 tags. DIRECT STs still not working; FEFLOW and TETGEN still not working; getsurface still not working

this leads to the other changes of this commit in the inheritance hierarchy
introduced a const_cast in Element::addNeighbor()

- added (a not yet finished version) CollapseMeshNodes programme
- changes in SimpleTests/MeshTests/CMakeLists.txt

[MeshCoarsener]
- new class used by CollapseMeshNodes
- can be used to make mesh nodes unique within a small epsilon radius

[MeshIO]
- added writing functionality for old mesh format

[GeoLib::Grid]
- changed interface (method getPointsWithinCube() -> getVecsOfGridCellsIntersectingCube())
- corrected documentation

[all mesh elements]
- added method clone()
- added method getType()
- removed attribute type from abstract base class Element