Add tests for add_title, add_units, add_precursors

[bpbond]

Also want to have checks that chunks have provided required documentation on their objects.
ALSO want to check that their precursors are in fact one of their inputs!
Put this logic in return_data or the driver?

  # Check that the chunk has provided required data for all objects
  for(at in c(ATTR_TITLE, ATTR_UNITS, ATTR_COMMENTS, ATTR_PRECURSORS)) {
    for(obj in names(dots)) {
       if(is.null(attr(dots[[obj]], at))) {
         warning("No '", at, "' attached to ", obj)
       }
     }
   }

[bpbond]

See #27

[bpbond]

Implemented in 3a11df8.