Single value cubes

Handle files written from Petrel that contain just a single value. The naive way of writing such files is to set min=max=value both for coding range and histogram. This triggers several corner cases and is not allowed in OpenZGY. But OpenZGY must do something reasonable when encountering such files.

The task is fairly large. Partly because a new property needs to be exposed from the accessor. And partly because support is needed both for C++ and Python and for several applications.

There are still things that might be improved, and still parts that have not been tested properly. See private/constcube-testplan.sh. I am committing the work done so far because I need to switch tracks.