Change the default k to 64, add 32 and 96

Hi, Michael. The default value of k of 25 is much too low for most data sets. This patch increases the default value of k to 64, and adds 32 and 96 as low- and high-k alternatives. We're looking into integrating KmerGenie or similar into the ABySS docker file. Have you considered integrating a k-choosing tool into Nucleotid.es upstream of all dBG assemblers?

This patch replaces #1—though I prefer #1 if you'll consider it, as it's more similar to our typical workflow with ABySS.