Make extract-terms, specificity, and sum-up-freq parallelizable

[parmentf]

extract-terms.js, specificity.js, and sum-up-freq.js use global variable (file-level) to perform accumulations (sums, counting, ...).

That implies that if used in a parallelized workflow (like, say, an ezs server), they may mix the countings, sums, of different inputs.

And that is not the intended way of use, since it will flaw the calculations.

Solution: integrate these global variables into the exported function's context (in its this), and initialize them when this.isFirst() is true.