Loop closure base

Here is part of the work of Tessa, former intern @PAL.

1. A base class for loop closure processor
2. A derived class for searching loop closure candidate based on frame covariance overlap

src/test/CMakeLists.txt

@@ -114,6 +114,10 @@ target_link_libraries(gtest_frame_imu ${PROJECT_NAME})
 wolf_add_gtest(gtest_pinhole gtest_pinhole.cpp)
 target_link_libraries(gtest_pinhole ${PROJECT_NAME})
 
+# ProcessorFrameNearestNeighborFilter class test
+wolf_add_gtest(gtest_processor_frame_nearest_neighbor_filter_2D gtest_processor_frame_nearest_neighbor_filter_2D.cpp)
+target_link_libraries(gtest_processor_frame_nearest_neighbor_filter_2D ${PROJECT_NAME})
+
 # ProcessorIMU class test
 wolf_add_gtest(gtest_processor_imu gtest_processor_imu.cpp)
 target_link_libraries(gtest_processor_imu ${PROJECT_NAME})
@@ -146,4 +150,4 @@ IF(OpenCV_FOUND)
 
   wolf_add_gtest(gtest_roi_ORB gtest_roi_ORB.cpp)
   target_link_libraries(gtest_roi_ORB ${PROJECT_NAME})
-ENDIF(OpenCV_FOUND)
\ No newline at end of file
+ENDIF(OpenCV_FOUND)