Adds two functions required for retrieving the anchors within an overlap.

…t calculation

to output_overlap_chains_by_backtrace.

… However, tests fail because the allocator used in testing can't actually allocate memory.

…meworks into anchor-backtrace-io

…e) and fix docs issues brought up in review.

…tride function.

… when instantiating device arrays.

…than CUB.

… to process backtraced overlaps.

- Use thrust transform_reduce and transform_exclusive_scan (rather than make_transform iterator) whe>
- Constify pointers where possible.
- Remove unnecessary includes.
- Tweak docs for backtrace_anchors_to_overlap

Primarily, this adds a number of tests for output_overlap_chains_by_RLE,
output_overlap_chains_by_backtrace, and backtrace_anchors_to_overlaps.
It also fixes a bug where the last anchor position in a chain was not returned
from backtrace_anchors_to_overlaps, causing faulty output when later extracting anchor
chains.

…meworks into anchor-backtrace-io