Rename BaseJO to BaseOpCode

Since we don't have for now a job definition object anymore, we rename
this class to BaseOpCode. It's still useful (and not merged with OpCode)
since it holds all the 'pure' logic (no custom field handling, etc.)
whereas OpCode holds opcode specific data (OP_ID handling, etc).

The patch also fixes the module's docstring.

Reviewed-by: imsnah