Misc Issues PR

[herbiebradley]

This PR contains several miscellaneous features:

• Minor linting and type hinting improvements, including making type hints of class labels consistently Union[int, str].
• Pin Rasterio to 1.1.8
• New _add_nodes function in geograph.
• Fix the merge_classes function by detecting neighbouring polygons of the same class and merging them. Closes #44
• Support for barrier_classes in add_habitat, with basic pathfinding functionality. This will only detect pathfinding problems if the polygon of the barrier node completely cuts the buffered polygon of the main node in half, preventing all access to the neighbouring polygon. This works by using Shapely's polygon difference function, see the comments for the actual algorithm.

The pathfinding code slows add_habitat down quite a bit, as might be expected, but it seems to make some difference to the final edge count:

When this one is merged I will create the 0.0.2 release, upload it to PyPi and add it to Zenodo.

[herbiebradley]

@Croydon-Brixton @rdnfn Thanks for the feedback - all issues have hopefully been resolved now! 🚀