Progress meter?

[bbimber]

This is low priority, but when we run nimble on big data, it logs the "Loading read sequences", and then it's a black hole for a long time. It might be nice if nimble could periodically log a 'progressed XXX sequences' type message. Ideally this would be connected to an optional command line argument for 'logInterval' or something, where maybe that defaults to 100K? Perhaps "-1" turns off all logging?