TUNING_CURVE_TO_RESPONSE_TYPE - get the response type ('mean', 'F1', etc) of a tuning curve document
[RESPONSE_TYPE,STIM_RESPONSE_SCALAR_DOC] = ndi.fun.stimulus.tuning_curve_to_response_type(S, DOC)
Given an ndi.document object DOC that is either a stimulus_tuningcurve or a
document that has a dependency, 'stimulus_tuningcurve_id', this function
looks up the 'stimulus_response_scalar' document and returns its
'response_type' field. This is typically 'mean', 'F1','F2', etc.