app/difficulty.py: adjust difficulty level to reflect test level

If a word appears in CET4 test, it has a difficulty level of 4.
If a word appears in CET6 test, it has a difficulty level of 6.
If a word does not appear in the above two tests, then its difficulty level is determined by its frequency.
     If this word is not in the frequency table, then its level is 1.
     If this word is in the frequency tables, then its level is determined as follows.
         log2( ((F+1) / (f+1)) ), where F is the largest frequency, and f is the word's frequency.
         If f=F, then log2(1) = 0, i.e., its difficulty level is 0.
         If f<F, then log2(1) is a positive value.

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