Sigma pr317

[JonathanGregory]

Fixes: #314

See issue #314 for discussion of these changes.

Release checklist

• [NA] Authors updated in cf-conventions.adoc?
• [NA] Next version in cf-conventions.adoc up to date? Versioning inspired by SemVer.
• [Y] history.adoc up to date?
• [NA] Conformance document up-to-date?

For maintainers

After the merge remember to delete the source branch.
Tags are set at the conclusion of the annual meeting; until then master always is a draft for the next version.

[johnwilkin]

I think this will work OK. Regardless of how the modeler orders the k index it is a universal convention that sigma, or s, increases upward from -1 at the bottom (or of c_depth) to 0 at the sea surface.