Disable "filter edges by weight" button even if the input filetype supports edge weights but has none given

[fedarko]

This is mostly an artifact of old GML graphs (e.g. the really big Shakya graph), but it's a good idea to get it taken care of regardless.

I guess, in the preprocessing script, if we identify a single node without edge weights given then we can just set some flag variable (and then a new column—let's call it has_edge_weights or something—in the database's assembly table) to FALSE.

[fedarko]

It'd also be very doable to implement this in the viewer interface, but I'd rather it be offloaded to the preprocessing script.