Update pmtiles code

[Robinlovelace]

With reference to nptscot/nptscot.github.io#258 and nptscot/nptscot.github.io#244 we have a plan to improve the experience for users when zoomed out. That is to:

• Change the existing pmtiles rnet generation so it only generates tiles up to zoom level 10 (as a starter for 10 to test the concept)
• Filter out the least important routes, e.g. keeping only the top 50% of segments by Go Dutch potential
• Create a new pmtiles file from this filtered smaller rnet dataset at zoom levels 10-5
• Combine the two .pmtiles files

That should solve it. Thoughts @mvl22 and @mem48 welcome.

[mem48]

These functions https://github.com/PlaceBasedCarbonCalculator/build/blob/main/R/pmtiles.R may be of interest. My make pmtiles R functions incling merging different sets if tiles together.

[Robinlovelace]

I feel a new itsleeds-hosted R package coming on...