checkPreferenceOrder: Check if preference order is complete
Description
This function checks if a given preference ordering is complete. If needed
it transforms the indices from R indices (starting at 1) to C++ indices
(starting at zero).
Usage
checkPreferenceOrder(pref)
Arguments
pref
is a matrix with a preference ordering
Value
a matrix with preference orderings with proper C++ indices or NULL
if the preference order is not complete.