This function is called by RankAggregator::consensusRanking.
The lower bound is the absolute lowest value a complete candidate ranking
could attain. Note, this value is not always achievable, so may differ
from the value returned by upperBound.
     For each pair of \code{Item}s, there are three possible calculations, depending
     on whether both \code{Item}s are in the \code{partial.ranking}, one is in
     and the other is out the \code{partial.ranking}, or both are not in
     the \code{partial.ranking}.
lowerBound(umat, partial.ranking)a matrix, which is either the output of evaluationMatrix,
or a subset of the output of evaluationMatrix.
a vector of rank positions
for each Item in umat that is ranked so far;
partial rankings may contain some - or all - NA
values.
A numeric value  for the lower bound of a partial.ranking