Implement fast load graph - Issue #152

[edoumazane]

Draft PR related to Issue #152

[edoumazane]

Tested locally on a subgraph with ≈ 200 vertices and ≈ 200 edges -> Wall time is down from 11 s to 1 s

• [x] implement and test on ClearMap.Analysis.Graphs.GraphGt.load
• [ ] implement and test on ClearMap.IO.IO.read
• [ ] implement unit test and test dats