[Example/OpenCV] Add OpenCV custom filter example: Average (#704)

[Example/OpenCV] Add OpenCV custom filter example: Average (#704)

* [Example/OpenCV] Add OpenCV custom filter example: Average

This patch newly adds an OpenCV custom filter example, which calculates
the average value of input tensor for each channel. The shape of the
input tensor is [N][y][x][M] and that of the output tensor is
[N][1][1][M].

This example is developed based on
nnstreamer_customfilter_example_average.c file.

Signed-off-by: Sangjung Woo <sangjung.woo@samsung.com>
[Touched up a few typos]
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>