Rewrite PCC.conflicts to make it more efficient

[Jean-Baptiste-Camps]

PCC.conficts is the more intensive function in the package, and has already concentrated much of profiling effort, but, we should see if we can make it more efficient, especially for the cases of alternateReadings.

Perhaps, for these, we could treat the full matrix to turn it into a list, containing lists for row, containing list for each cell, instead of splitting it on the fly.