[IO] Remove allRastersExists() from interface.

FileIO/AsciiRasterInterface.cpp

@@ -216,7 +216,9 @@ void AsciiRasterInterface::writeRasterAsASC(GeoLib::Raster const& raster, std::s
     out.close();
 }
 
-bool allRastersExist(std::vector<std::string> const& raster_paths)
+
+/// Checks if all raster files actually exist
+static bool allRastersExist(std::vector<std::string> const& raster_paths)
 {
 	for (auto raster = raster_paths.begin(); raster != raster_paths.end();
 	     ++raster)

FileIO/AsciiRasterInterface.h

@@ -56,10 +56,6 @@ private:
 /// otherwise the returned vector contains pointers to the read rasters.
 boost::optional<std::vector<GeoLib::Raster const*>> readRasters(
     std::vector<std::string> const& raster_paths);
-
-/// Checks if all raster files actually exist
-bool allRastersExist(std::vector<std::string> const& raster_paths);
-
 }
 
 #endif /* ASCIIRASTERINTERFACE_H_ */