call_vision_api         helper function to send POST request to the
                        Google Vision API
create_request_body     helper function to create json for response
                        request
create_single_image_request
                        helper function to create a list of details of
                        one image annotation request
encode_image            helper function to base64 encode the image file
extract_annotations     helper function code to extract the annotations
extract_error           helper function code to extract error from API
                        response into a data.table
extract_response        helper function code to extract the response
                        data.frame
extractor               helper function code to provide an extractor
                        function for different feature types
face_detection_extractor
                        helper function code to extract API response
                        into a data.table for given feature type
gcv_get_available_feature_types
                        helper function code to record available
                        feature types
gcv_get_image_annotations
                        Get parsed image annotations from the Google
                        Cloud Vision API
gcv_get_raw_response    Get raw API response from the Google Cloud
                        Vision API
gcv_get_response        helper function to call the API for one batch
                        of images
get_bounding_boxes      helper function code to extract Bounding Box
                        x,y coordinates for an API response element
get_invalid_image_paths
                        helper function to validate input image paths
label_detection_extractor
                        helper function code to extract API response
                        into a data.table for given feature type
landmark_detection_extractor
                        helper function code to extract API response
                        into a data.table for given feature type
logo_detection_extractor
                        helper function code to extract API response
                        into a data.table for given feature type
ocr_extractor           helper function code to extract API response
                        into a data.table for given feature type
split_to_chunks         helper function to split a vector to
                        approximately equally sized chunks
