A subgroup of identical files, typically smaller than a FileGroup.
A subgroup is formed by files sharing the same path prefix, e.g. files on the same volume.
In terms of file deduplication activities, a subgroup is an atomic entity -
all files in a subgroup must be either dropped or kept.
Groups identical files together by 128-bit hash of their contents.
Depending on filtering settings, can find unique, duplicate, over- or under-replicated files.
Sort files so that files with highest priority (newest, most recently updated,
recently accessed, etc) are sorted last.
In cases when metadata of a file cannot be accessed, an error message is pushed
in the result vector and such file is placed at the beginning of the list.