Add word part frequencies to decompounder tree

[HannaLindgren]

By adding a frequency number to the compounding word parts, this can be used to select a best guess when several alternatives with the same number of compound parts are found.

Furthermore, very high frequent suffixes can be used for determining compound boundaries, although the prefix is not known (e.g., in Swedish "-gata" (street) may be such a safe bet).

[moved here from Phabricator Wikispeech]