Tighten SWC requirements, comments. (#1170)

* Remove contiguous numbering requirement from 'strict' mode SWC
parsing, as it is not necessary for correct interpretation as a segment
tree.
* Describe in the swcio.hpp header how (strict) swc data is interpreted
as a segment tree.
* Demonstrate in unit tests that out-of-order and non-contiguously
numbered records are valid for `parse_swc`.
* Add unit test for `as_segment_tree`.