Unified mesh layer mapping

* Unifies the interface of MeshLayerMapper and LayeredVolume and introduces base class for these two  to handle common functionality.
* Allows to create prism-meshes from incomplete layers or conflicting information and unifies/simplifies GUI dialogue for inputting the relevant information.
* Moves ascii-raster I/O to FileIO
it seems TetGen uses 10^-8 as default episolon for gemetric calculations. The eplison can be changed by -T option. see 
http://wias-berlin.de/software/tetgen/1.5/doc/manual/manual005.html#hevea_default148

In the code, I found another option --TT which can also configure another kind of epsilon. but i'm not sure where it is used.