Detailed guidelines for shell scripts

New scripts should follow this standard, except where dependence on legacy scripts prevents this; legacy scripts will be migrated over a period of time according to a managed programme.

• 1. When (not) to use shell
• 2. Which shell to use
• 3. Preferred shell options
• 4. Google Shell Style Guide
• 5. ecFlow task wrappers
• 6. Use of shellcheck and shfmt
• 7. Available commands and dependencies