exploded-tree improvements

[jameshadfield]

Context
A list of improvements to be made to the multi-tree / exploded-tree functionality introduced in #1442

Description

• "Zoom to selected" fundamentally doesn't play nicely when viewing subtrees, due to the way we calculate the links between in-view tips. We'll revisit this concept shortly with the accordion zoom & other ways of zooming the tree.
• Connecting lines (e.g. between the subtree root and it's parent in the unexploded tree) are for the next PR, however the tree layout knows about this link as it's used to compute the subtree stem length.
• URL state is not implemented, as I wanted to wait for more testing here
• Subtrees are separated in Radial & Unrooted views, however this separation is expressed by (i) a change in angle and (b) the distance from the subtree root to the overall tree root (i.e. divergence or time). Therefore if each subtree root has a similar origin divergence/time, then there will be little to no separation no matter what the angle is! Plenty of work can be done here (see commit messages for more).
• Internal branches with no tips (because their children have all have been pruned) are still displayed. This isn't ideal, but is an edge case and will be tackled along with the connecting lines as they have a lot of overlap.
• The helper functions for "get parent of node" need to be tidied up and changed throughout the code. For most use cases we want to return undefined if the node is a subtree root. The one (?) exception is for layouts which want to know the "original root" to calculate their stem length. The scatterplot, at the least, is not correct here (e.g. see c94eb60).
• If all internal nodes are unannotated for a trait, it shouldn't be available as an option