scan: use natural sort order with --sort opt

[oliver-sanders]

Use natural sort order with cylc scan.

Place function in generic module for easy reuse.

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.py and
  conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required.

[wxtim]

• Read
• Cylc Scan --sort appears to work
• Poked the code

The only extra I can think of is including an example where the strings look like Python Hex, Oct and Binary numbers, but not essential.