Make changes necessary to convert rgb images to aron format

When using ndarray_to_aron() (in python3-armarx/armarx_memory/aron/conversion/ndarray/common.py) to convert rgb images, is_point_cloud() would return true because the images have fields. To prevent that an additional condition for a ndarray to be classified as a point cloud was added. Also the function convert_int8_to_dtype_rgb() to convert images to an aron format was added.