A simple phrase property class to access the three phrase count values.

The counts are usually not needed during decoding and are not loaded
from the phrase table. This is just a workaround that can make them
available to features which have a use for them.

If you need access to the counts, copy the two marginal counts and the
joint count into an additional information property with key "Counts",
e.g. using awk:

$ zcat phrase-table.gz | awk -F' \|\|\| '  '{printf("%s {{Counts %s}}\n",$0,$5);}' | gzip -c > phrase-table.withCountsPP.gz

CountsPhraseProperty reads them from the phrase table and provides
methods GetSourceMarginal(), GetTargetMarginal(), GetJointCount().