Replacement date-aware spiral visualisation

[lentinj]

Replace the existing spiral visualisation with one that is date aware, i.e. instead of one "segment" of a logarithmic spiral per branch, we vary segments based on a node's date relative to it's surroundings.

• Spiral gets described by a custom logarithmic spiral shape, tapering from start-node size to end-node size.
  • Spiral object drawn by segmenting the shape into separate chunks, which can be individually coloured
• Time gets mapped by adding length to spiral (a bit like adding several chunks of the existing spiral), and colour changes as you go when you move between different geological periods. Maximum length of spiral ~360deg.
• Tapering for tree-branching only affects the outside edge of the spiral, which gets offset to allow for branch, smoothly ending up at the same end taper as if branching never happened. Unlike previous spiral, keep spiral nodes in node centre rather than offsetting. Inside edge is geometric spiral construction line, origin is inner edge of parent node
• Branches coming off would spiral in opposite direction, outer edge of parent node is spiral construction line,

Later ideas to experiment with:

• Bezier curves would be assigned for outside/inside edge of shape to make it a smooth spiral.
• "Kink" at start of branching off. @jrosindell was thinking that the 'tick' to the left should still be a log spiral object to the left it's just that it will have to join onto a right log spiral if it needs to continue on.
• Compressing spiral when there's no branching, to keep colour changing but not lost.

[lentinj]

Here's a diagram from last week that looks worth keeping: