Calculate sequence read depth from GFA files if possible

[fedarko]

Figure out how to do it and do it (by just adding the depth info to the .db file).

[fedarko]

Ah, so it looks like depth is calculated using the "read count" optional parameter (RC:i:[some int here]) as the integer in the RC divided by the length of the sequence. Or at least that's how Bandage does that. If a sequence doesn't have a read count parameter then its depth is assumed as 1.

Since I'm not entirely sure what the implications of this optional parameter are or of assuming a uniform read depth of 1 for nodes, I think I'm going to stick with just giving all GFA sequences a "N/A" depth in the viewer for now. I can come back to this later and reevaluate my approach, but there are more important things to focus on for now.

[fedarko]

This issue was moved to marbl/MetagenomeScope#23