Making RawSubFile usable from Python (#158)

- Removed a printout left from debugging
- return also header when reading
- added read_n 
- check for error in ifstream

CMakeLists.txt

@@ -388,7 +388,9 @@ set(SourceFiles
     ${CMAKE_CURRENT_SOURCE_DIR}/src/RawFile.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/src/RawSubFile.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/src/RawMasterFile.cpp
+
     ${CMAKE_CURRENT_SOURCE_DIR}/src/utils/task.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/utils/ifstream_helpers.cpp
 )