[NEW_FEATURE] Allow faster graph loading when edge geometry is not needed

[edoumazane]

Complete mouse brain vascular graphs can be slow to load (≈ 2 minutes)

• Most of the size of ClearMap GT files is due to the "edge_geometry" properties.
• In ClearMap.Analysis.Graphs.GraphGt.Graph objects, "edge_geometry" properties are stored as "graph_properties"
• graph_tool.load_graph allows to optionally ignore a list of properties

A useful feature would be to optionally specify properties to ignore in ClearMap, when they are not needed.