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.