Add grouped parallel I/O for graph placements of unpaired reads

[Sagorikanag]

Summary

Adds parallel grouped single-read alignment iteration for multiple graph placements sharing the same read name.

This will be needed by vg surject --diploid-map, where the primary and secondary graph placements for a read must be processed together.

Changes

• Add a generic grouped_for_each_parallel iterator.
• Add grouped GAM input using ProtobufIterator.
• Add grouped GAF input by query name.
• Perform expensive GAF-to-Alignment conversion on worker threads.
• Return the number of processed read-name groups (runs).

Input requirement

Records belonging to the same read must be consecutive in the input.