Upgrade computeFrequecies to python3
Upgrade computeFrequecies to python3

This brings noticeable improvements, both in terms of memory and
execution time. It should be related, among other things, to the
new "Key-Sharing Dictionary" feature.
(See https://www.python.org/dev/peps/pep-0412/)

lucach/spellcorrect,lucach/spellcorrect,lucach/spellcorrect,lucach/spellcorrect