Provide a convention for boundary variables for grids whose cells do not all have the same number of sides

[JonathanGregory]

See issue #163 for discussion of these changes.

Release checklist

• [NA] Authors updated in cf-conventions.adoc? Add in two places: on line 3 and under .Additional Authors in About the authors.
• [NA] Next version in cf-conventions.adoc up to date? Versioning inspired by SemVer.
• [Y] history.adoc up to date?
• [Y] Conformance document up to date?

[ChrisBarker-NOAA]

LGTM -- thanks!