Add a diffing function...For now the diff function uses the Linux tool "diff" to generate the diff
since it is efficient and straightforward.
* gn3/computations/diff.py (generate_diff): New function.
* tests/unit/computations/test_diff.py: Test cases for ☝🏾.
Rename file_utils to fs_helpers...Generally avoid naming things with a "utils" prefix/ suffix since it
encourages contributors to dump any new functions there; and over time, as the
code grows, things get messy...