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
Just tried to look up the float thing and you are right about TetGen using double. Still, it's hard to debug and I don't know if the problem is in my code or something TetGen-related :(