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
10^(-7) is because of tetgen only using floats.
if you need to map stratigraphic layers with nanometer thickness you won't be able to use tetgen.