refactor Subfile::get_frame_into to use utils::merge_frames

[Bechir-Brahem]

network_io and file_io both require the same functionality of merging frames according to a specific geometry.

we should use the merge_frames function (in utils) for both network_io and file_io. this improves testability and avoid redundancy in a very error prone functionality.

currently only network_io uses this function (#65)