Complex shared paths with backward and forward parts with non-existing junctions

[mattijn]

This PR fix #63.
The issue was twofold:

• Firstly, shared paths containing both forward and backward elements, where the forward or backward elements are non-contiguous could not be directly loaded as MultiLineString (this: MultiLineString([LineString, MultiLineString]) doesn't work in shapely)
• Secondly, in the case a LineString has partially shared paths with non-existing junctions. Then these are inserted before splitting. These non-existing junctions are rightly detected, but were not sorted in the right order before insertion. Leading to problems in the Dedup phase. Fixed in Cut phase.

Last, the .to_svg() function was improved so one can use .to_svg(separate=True, include_junctions=True) during debugging.

Like:

from topojson.core.cut import Cut
data = [
    {"type": "LineString", "coordinates": [(0, 0), (10, 0), (10, 5), (20, 5)]},
    {
        "type": "LineString",
        "coordinates": [
            (5, 0),
            (25, 0),
            (25, 5),
            (16, 5),
            (16, 10),
            (14, 10),
            (14, 5),
            (0, 5),
        ],
    },
]
c = Cut(data)
c.to_svg(separate=True, include_junctions=True)