ID space is not compacted

[edawson]

lasso uses the IDs as a map(basepair : node) in the graph. This unfortunately means that our ID space across multiple contigs quickly grows, and that there are collisions in the GFA output if we have multiple variant contigs (though we could construct each contig graph individually and then join them with a vg ids -j).

We should ideally maintain our map outside of ID space and then increment IDs by one every time a new node is added. This is in line with what VG does.

[edawson]

Fixed.