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This makes edge deletion O(1) if fast edge deletion is enabled for the
graph.

This enables the acceptance of numpy types such as numpy.int64 as
parameters to functions taking C++ integers or floats.

This also includes a "mid_marker_pos" edge attribute which allows a
relative placement of the mid marker along the edge.

Avoid hash of pair<vertex_t, vertex_t> types.

This greatly improves the performance of the previous algorithm, and
includes support for hierarchical models.

The filtered array returned now also reflects the vertex filtering, so that
it corresponds only to actually visible edges in the graph, not only
those explicitly selected via an edge filter.

This also includes the new text_rotation, and text_offset properties.