Creates a vector of labels from a parameter-condition combination matrix, resulting from a call to prms_cond_combo. Used, for instance, in coef.drift_dm.
Value
A vector of labels with as many entries as the columns of
prms_cond_combo had, combining parameter and condition (if necessary).
If the parameter labels are already unique (because all parameters do not vary across conditions or are selectively used for one condition), then only these parameter labels are returned
