New function pgr betweenness centrality

[bedupako12mas]

This PR adds one new pgRouting function:

• pgr_betweennessCentrality: Returns the relative betweenness centrality of all vertices of a graph.

This is a final merge PR containing commits from several different PRs combined.

The intermediate PRs made on the GSoC-pgRouting repository are as follows:

Description	Pull Requests
pgr_betweennessCentrality	pgRouting/GSoC-pgRouting#378 pgRouting/GSoC-pgRouting#383 pgRouting/GSoC-pgRouting#386 pgRouting/GSoC-pgRouting#388 pgRouting/GSoC-pgRouting#390 pgRouting/GSoC-pgRouting#392 pgRouting/GSoC-pgRouting#393 pgRouting/GSoC-pgRouting#396 pgRouting/GSoC-pgRouting#398 pgRouting/GSoC-pgRouting#399 pgRouting/GSoC-pgRouting#402 pgRouting/GSoC-pgRouting#403

[cvvergara]

Please wait until all the mentors approve to rebase. Do not merge, do a rebase. Add the link to this PR to your final report.