Update template and schema

[pont-us]

The Black Sea GeoJSON is being modified at deepesdl/cube-gen#7 in ways that don't conform to the current template and schema. The template and schema need to be updated to keep pace with these modifications.

[forman]

As the sepc has just been updated once more, there is an urgent need to

1. update template and schema
2. write a Python function + CLI that does the validation of a cube.geojson against spec and schema (Need a cube validation tool #7)

[forman]

We need to think about #5.
The spec and the convention both currently lack a definition of processing_steps.