F0_F1_RESPONSES - get the F0 and F1 responses for a tuning curve document
[F0,F1] = F0_F1_RESPONSES(S, DOC, [STIMULUS_INDEX])
Given an ndi.document object of type stimulus_tuningcurve (or a document that has a single
dependency of 'stimulus_tuningcurve_id'), and a response_index number (the entry in the 'mean'
response array of responses), this function returns the f0 and f1 responses for that stimulus.
This function looks up the appropriate stimulus_tuningcurve objects.
If STIMULUS_INDEX is not given, then the overall maximum response rate (either mean or F1)
is examined and that location is taken as the STIMULUS_INDEX.