a function for by-kid, by-item cdi info

Often we run eye-tracking studies examining infant word comprehension. It would be useful to cross-check our collected data against parent report with a function that works as follows:

For each item in an eye-tracking study, this function would provide each child's CDI value (understands, produces, neither, not on CDI) for each item.

proposed output:

| subj | target | CDI |
|-----|--------- |---------------|
| s01 | apple | produces |
| s01 | shang | NA |
| s02 | banana | produces |
| s02 | train | understands |