Support conditional generation of different graph view types

[fedarko]

Similarly to how -nodna allows the user to not pass sequence data to the .db file generated by collate.py, we should support features where the user can pick and choose which graph types to generate layouts for. Right now there's just two choices (double graph with variants detected vs. single graph with SPQR tree integration), but I imagine if/when we add the functionality of just normal single graphs without SPQR tree integration (a la #10) we should probably let the user choose which types to include in the .db file.

Also worth noting that GMLs don't really have a distinct single graph. Not sure if we should still lay out the undirected version of the graph, or just block that feature for all files with already-oriented contigs. Something to think about.

[fedarko]

This issue was moved to marbl/MetagenomeScope#49