Improve display of circular markers designating path nodes.

[dbrnz]

See https://www.wrike.com/open.htm?id=1005437450 for background discussion -- questions have been raised about the usefulness of the specific anatomical structures map features, seen as the cyan circles below.

These features are present as end-points for SCKAN paths but could be effectively hidden from users by excluding them from rasterisation when creating image tiles. The nodes would still be available for path connections and show on mouse over, but otherwise not appear.

All the nodes shown (cyan (specific structure), pink (tissue region) and green (ganglia), as well as the others in the new legend (brain nuclei and nerve plexus), could in fact be excluded from the rasterised image and instead have type information added as a property for the viewer, so that their visibility and colour can be controlled with MapLibre styling rules -- this is necessary if/when we add the ability to show/hide nodes, and is part of adding more semantic markup to generated maps (and the FC work).