FileIO -> MeshLib::IO.

In order to make OGS more modular:
- The {read/write}Mesh{From/To}File.{h,cpp} are moved from FileIO to MeshLib/IO.
- The FileIO/Legacy folder was moved to MeshLib/IO/Legacy.
- Include paths are changed according the changes above.

When building shared libraries -DBUILD_SHARED_LIBS=On must be passed to the
cmake invokation. Otherwise the default static linking applies.

This reduces the compile time, which is especially interesting for the debug
builds.

Use instead layered mesh constructor taking a GeoLib::Raster.
Reading of the raster happens inside the application now.

Those are automatically generated.