Subsurface meshes via TetGen

* 'master' of https://github.com/ufz/ogs: (308 commits)
  fix a compiling error in MSVC
  Small fix for the ninja build system.
  fix compiling errors in MSVC
  Fix NumLibFemIsoTests.
  Fixed indentation.
  [MeshLib] Fixed unused variable warnings.
  recovery two files that are corrected in one PR by Tom
  Remov more warnings when g++ is used
  [TetGenInterface] Made two methods const.
  [TetGenInterface] Removed unused code.
  [TetGenInterface] alloca is not part of standard.
  [TetGenInterface] parseElements(): const method.
  [TetGenInterface] Narrowed scope of pos_beg and pos_end.
  [TetGenInterface] Var "id" is not used.
  Style: Removed spaces, tabs at the end of line.
  fixed indentation
  added functionality for writing tetgen poly files
  added read method for tetgen geometries
  catching comments or empty lines inbetween nodes or elements
  removed class variables for nodes and elements and did some small optimisations
  ...

Fix a compiling error in MSVC.

See http://docs.opengeosys.org/docs/devguide/advanced/build-with-ninja

Add FE type TRI3

fix compiling errors in MSVC

Remove more warnings when g++ is used

TetGen geometry import/export

Fix Tests, Remove warnings