add khmer 2.x compat functions back in

[ctb]

it would be nice to be able to use the next release of khmer with the current version of sourmash, and I think right now sourmash depends on khmer.load_nodegraph which doesn't exist in master. maybe we could put that back into khmer?

[luizirber]

we can also do

- self._data = khmer.load_nodegraph(f.name)
+ self._data = khmer.Nodegraph.load(f.name)

(from this PR: https://github.com/dib-lab/sourmash/pull/395/files#diff-0b5baa8c66f625d5b16a6ada6c9aa998R681)

[standage]

Yeah, I think it would be clean and easy to add a clearly marked "backwards compatibility" section to the khmer/__init__.py file with a few such definitions. Let me know if there are any others.

[camillescott]

I'm -1 on this. It should be on external libraries to route around deprecated APIs.

[luizirber]

We got around this issue in sourmash with sourmash-bio/sourmash#508