v1.7.1 - Export/Import: eliminated duplicate writes

UPDATE

1. Do not write contraction duplicates into CSV-file
2. Do not write edges duplicates into CSV-file
3. Update Graph struct:
   replaced

// ...
contracts    map[int64]map[int64]int64
// ...

with

// ...
type ContractionPath struct {
	ViaVertex int64
	Cost      float64
}
// ...
contracts    map[int64]map[int64]*ContractionPath
// ...

For more infromation look into this PR: #16