Rename `rnet_merge` as `rnet_merge_sum` and add settings to reproduce NPT results

[Robinlovelace]

As discussed with @wangzhao0217, please give this a bash if you get time.

Only after 2025-01-01 data complete though, this is a nice to have (and will mean we can delete some lines of code if implemented upstream in stplanr).

See https://github.com/nptscot/geodataprep and commits for details.

[rdesfo]

https://github.com/nptscot/geodataprep returns a 404. Should this issue still be open?

[Robinlovelace]

Should this issue still be open?

Yes, the issue still stands, it's probably worth documenting in a vignette.

That's a private repo because it processes and contains some non-open data, hope to add more details here.