Add CSI index support

[valeriuo]

We had a request from a BCFtools user to assist in splitting the chromosomes from a VCF file into smaller scaffolds, which can then be indexed in TBI format. The user intends to use the indexed VCF file in a PopGenome analysis, but currently cannot do so, because some chromosome in the file are larger than 2²⁹-1, which is the upper limit of the TBI format, and PopGenome apparently does not accept CSI indices, which could accommodate these large chromosomes.

You can find a minimal description of the CSI format here.