quired for "start" action
                                 Every scan object is either a string descriptor or an object:
     [
       "descriptor",             (string) An output descriptor
       {                         (json object) An object with output descriptor and metadata
         "desc": "str",          (string, required) An output descriptor
         "range": n or [n,n],    (numeric or array, optional, default=1000) The range of HD chain indexes to explore (either end or [begin,end])
       },
