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
doesn't work.
I assume numeric limits for float has an eps of 10^-8 but tetgen gives an error msg that points are identical and refuses to mesh. it worke with 10^-7 though for the examples i tested it with.