EPSG conversion functionality

Repurposes io.osm_graph_from_poly_wgs to io.osm_graph_from_poly:

• The input polygon's projection no longer has to be 4326 WGS, though this remains the default. It is now possible to specify other EPSG codes, e.g. 27700 if working with BNG.
• A to_epsg_code parameter has been added for specifying the output EPSG code. This is an optional parameter, and the behaviour will default to the local UTM CRS as per previous behaviour. If specified, the generated street network will be returned in the specified CRS.

Adds nx_epsg_conversion for converting networks from between specified EPSG CRS codes.

Refreshes docs and examples accordingly.

Minor tweaks and edge case handling in graph cleaning methods.