C++ implementation of pgr_kruskal

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• Fix pgr_kruskal file

Signature

seq | edge| cost| tree_cost

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Output for edge table:

 seq | edge | cost | tree_cost 
-----+------+------+-----------
   1 |    1 |    1 |         1
   2 |    3 |    1 |         2
   3 |    7 |    1 |         3
   4 |   15 |    1 |         4
   5 |   18 |    1 |         5
   6 |   17 |    1 |         6
   7 |   14 |    1 |         7
   8 |   16 |    1 |         8
   9 |   13 |    1 |         9
  10 |   12 |    1 |        10
  11 |   11 |    1 |        11
  12 |   10 |    1 |        12
  13 |    6 |    1 |        13
  14 |    2 |    1 |        14
(14 rows)

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when sub_graph is added then result:

 seq | sub_graph | edge | cost | tree_cost 
-----+-----------+------+------+-----------
   1 |         1 |    1 |    1 |         1
   2 |         1 |    3 |    1 |         2
   3 |         1 |    7 |    1 |         3
   4 |         1 |   15 |    1 |         4
   5 |         1 |   14 |    1 |         5
   6 |         1 |   16 |    1 |         6
   7 |         1 |   13 |    1 |         7
   8 |         1 |   12 |    1 |         8
   9 |         1 |   11 |    1 |         9
  10 |         1 |   10 |    1 |        10
  11 |         1 |    6 |    1 |        11
  12 |         1 |    2 |    1 |        12
  13 |         2 |   17 |    1 |         1
  14 |         3 |   18 |    1 |         1
(14 rows)