support variant graphs

[phiweger]

https://github.com/vgteam/vg

[phiweger]

represent graph in JSON:

• https://github.com/jsongraph/json-graph-specification/blob/master/json-graph-schema.json
• https://github.com/jsongraph/json-graph-specification

[phiweger]

note: some collections of viruses are so diverse, that there is no real gain in storing them in a variant graph. so both - a flat sequence as well as a variant/ assembly graph' should be supported, where a linear sequence can be seen as a graph with one node and no edges.

also, a valiant graph has a 1 : n mapping (one graph, n documents) while has assembly is a 1 : 1 mapping.

[phiweger]

basically assume the sequence field to be a graph and offer corresponding functionalities (annotations need to hold the corresponding coordinates).