* Rename `as_segment_tree` to `load_swc_arbor`.
* Remove `swc_mode`: `sort_and_validate_swc` now performs the `relaxed` checks by default, `load_swc_arbor` performs the `strict` checks. 
* Change `load_swc_xxx` to only accept `swc_data` as an argument
* Remove default constructor for `swc_data` and enforce validation of SWC records via `sort_and_validate_swc` at construction.
* Add fixes for docs and unit tests.