Remove "-d"rop flag in favour of a new option

[benducke]

Any model except "complete" might leave some input points disconnected from the result network (let's call them "isolated nodes"). Currently, such nodes can be dropped from the nodes output map using flag "-d". However, these nodes present an interesting challenge that could be addressed in a number of ways, using a second connect pass, e.g.:

• connect each isolated node to closest connected node variations: either connect to only originally connected nodes (connect least distant one first), or also to previously isolated nodes
• connect each isolated node to closest link segment variations: as above, for previously existing vs. newly created segments
• connect each isolated node to closest node or link segment variations: as above It might be prudent to issue a warning message each time an isolated node is connect to the existing network. Also, when connecting to a link segment, a new node might be created on the intersection.

[benducke]

Related:

31 (output intersection points)

6 (automatically connect isolated nodes to reachable link segments)