Added an example calibration file for the logitech C270 camera with a resolution of 640x480

calibration/logitech_c270_640x480.yaml

+image_width: 640
+image_height: 480
+camera_name: narrow_stereo
+camera_matrix:
+  rows: 3
+  cols: 3
+  data: [811.587917, 0.000000, 316.684131, 0.000000, 809.975436, 232.835325, 0.000000, 0.000000, 1.000000]
+distortion_model: plumb_bob
+distortion_coefficients:
+  rows: 1
+  cols: 5
+  data: [-0.016863, 0.026776, -0.005160, 0.000093, 0.000000]
+rectification_matrix:
+  rows: 3
+  cols: 3
+  data: [1.000000, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000, 0.000000, 0.000000, 1.000000]
+projection_matrix:
+  rows: 3
+  cols: 4
+  data: [811.962280, 0.000000, 316.721055, 0.000000, 0.000000, 809.312805, 231.520901, 0.000000, 0.000000, 0.000000, 1.000000, 0.000000]