replace khmer with oxli

[dr-joe-wirth]

Is your feature request related to a problem? Please describe.
khmer is great, but it is no maintained and has an incompatible conda installation

Describe the solution you'd like
oxli was developed to add the counting functionality that khmer provides. Currently, it does not have any way to retrieve kmers, but I have raised this as an issue

Describe alternatives you've considered
There really aren't any other python packages that count kmers efficiently. I may use a different package for retrieving kmers if oxli never implements one.