`pollen_data_gen`: Lift precompute

[anshumanmohan]

The precomputation done inside depth was written before mygfa (or slow_odgi) was a thing, but I suspect there is some duplication going on. Even if not, certainly some of that computation can be lifted into mygfa/precompute so that other commands can use it. Case in point, this lifting should happen before starting work on #96 , so that the expanded simple can use the computation that currently lives in depth.

• [x] See if there is duplication between pollen_data_gen depth and mygfa/preprocess. Cut down on any.
• [x] Lift computation from pollen_data_gen depth to mygfa/preprocess.

[anshumanmohan]

Just took a look, and I don't really see an opportunity. There may be lifting within the pollen_data_gen module, but I don't think anything else from here needs to be lifted all the way out of this module and into the mygfa module.