Improve the assumption made by merge nearest network

[virio-andreyana]

@ekatef @davide-f

Feature request

This is the raw results of the function merge_into_network() in simplify network.py. This didn‘t solve the issue because

• it overconnects in one region (Indonesia, Singapore)
• It still left some regions fully disconnected (myanmar, cambodia)
  There must be a logical assumption where buses connects to the nearest network

Originally I would like to create a function that does similar to merge_into_network() with this assumption

• The bus in question must be a stub or an isolated node
• It must have a tolerance radius to not overextend
• Do not connect to the same subnetwork
• Do not go through waters
• Do not go across national border

This is different from merge_into_network() because

• Stubs are removed before merge_into_network() are run, not after
• The use of tolerance radius in gpd.sjoin_nearest()
• Not bypassing the offshore shapefiles

The rest behave quite similar.

I tried to create this code myself, but it always causes novel error in the use of busmap with minimal impact on connections. I was wondering if you can improve this.