Add codes in labelling to ensure tiles inside bbx

## Background

A bug was recorded in #176. In my testing, the tiles written in the label files after running the code `label-maker labels` are partly out of the bounding box provided inside the config file. I have added a checking function for this situation.

## Changes
Inside the file label.py add the following function right before the function make_labels

Inside the function make_labels, update the variable `tile_results` before writing out labels

This would ensure the file `labels.npz` and `classification.geojson` not contain any tiles outside the bbx