ENH: devise method for intelligent assignment of `unobserved_weight`

[nbokulich]

I am seeing some cases where unobserved_weight's default setting (1e-6) is actually higher than some observed weights (of very rare species obviously).

It would be useful to automatically set this based on some intelligent approach. Some ideas:

1. set to the lowest observed weight (or some value below that?)
2. set to 1 / N where N = the total number of sequences observed?

Maybe this does not really substantially impact performance, but I wonder if it could impact classification of rare SVs.