Detect other types of structural patterns

[marbl]

Detect other types of structural patterns

[fedarko]

also: highlight & remove (?) dead ends -- e.g. scaffold 4939 in Shakya component 4.

[fedarko]

Highlight all A-B-A cycles, even with other incident edges on B/A -- have them take priority over bubbles

[fedarko]

Include types of structural variants/etc. in the assembly/component info -- give the user a summary of information.

also include the facility to collapse all of a certain type of structural variants.

[fedarko]

Detecting 3-node bubbles should be taken care of by the SPQR code (#71). So the main TODOs for this issue are dead ends and "looser" cycle detection. The other two structural pattern things mentioned in above comment should (edit: now, are) be their own issues.