Add "overwrite" functionality to Python API

[hyanwong]

I'm getting bitten when rerunning small scripts because I can't overwrite zarr files, e.g.

import bio2zarr.tskit as ts2z
ts2z.convert("input.trees", "output.vcz")  # can only be run once 

I don't want to create these in a temp directory, as I want them hanging around, but am happy to overwrite them when re-running. So could we have an "overwrite" parameter?

ts2z.convert("input.trees", "output.vcz", overwrite=True)

[jeromekelleher]

Good call.

[jeromekelleher]

To implement this I think we'd abstract out the code in the cli to delete a path into core.py, and push that functionality into all the separate convert functions.