Improve coding style and readability
Add a format_version attribute to the description meta data

The format_version attribute indicates the version of the JSON data format.
For now Machinery only supports system descriptions of the current
format_version, there is no migration path for old formats.
Raise error if the user tries to load a description with old data format
Marks descriptions with incompatible data format in "machinery list"
Describe format version policy in the data model description