[feature] variants of tf-idf weighting

Closes #8 (closed)

Implements three different variants of tfidf, while the main differences are in the calculation of the tf.

1. natural: tf x log(N/df)

2. logarithmic: log(1+tf) x log(N/df)

3. augmented: (0.5 + (0.5 * tf) / max(tf))) x log(N/df)

Even thought natural and logarithmic have similar performance, the results from the augmented variant looks very off. On a closer look we can see that most sim values are between 0.8 and 1.

output/mongoElkanJaro-07-2020Jun01-1659.csv

+threshold,precision,recall,fMeasure,allPositive,truePositive,perfectCompleteClusterNo
+0.0,0.06188340807174888,0.053036126056879324,0.05711920529801324,1115,69,3474
+0.05,0.06818181818181818,0.05764796310530361,0.06247396917950852,1100,75,3489
+0.1,0.05850091407678245,0.04919292851652575,0.05344467640918581,1094,64,3495
+0.15000000000000002,0.06600361663652803,0.056110684089162186,0.06065641877856253,1106,73,3483
+0.2,0.06974637681159421,0.05918524212144504,0.06403326403326402,1104,77,3485
+0.25,0.059513074842200184,0.05073020753266718,0.05477178423236515,1109,66,3480
+0.3,0.07285974499089254,0.061491160645657184,0.06669445602334306,1098,80,3491
+0.35,0.07239819004524888,0.061491160645657184,0.06650041562759766,1105,80,3484
+0.39999999999999997,0.07942238267148015,0.06764027671022291,0.07305936073059362,1108,88,3481
+0.44999999999999996,0.06588447653429604,0.056110684089162186,0.060606060606060615,1108,73,3481
+0.49999999999999994,0.062100456621004566,0.05226748654880861,0.05676126878130217,1095,68,3494
+0.5499999999999999,0.06407942238267147,0.054573405073020755,0.05894562058945621,1108,71,3481
+0.6,0.0683453237410072,0.05841660261337433,0.06299212598425198,1112,76,3477
+0.65,0.05772646536412078,0.04996156802459646,0.053564070869386075,1126,65,3463
+0.7000000000000001,0.05972850678733032,0.05073020753266718,0.05486284289276808,1105,66,3484
+0.7500000000000001,0.060414788097385035,0.05149884704073789,0.05560165975103735,1109,67,3480
+0.8000000000000002,0.07553956834532374,0.06456571867794005,0.06962287608785743,1112,84,3477
+0.8500000000000002,0.060798548094373864,0.05149884704073789,0.055763628797336665,1102,67,3487
+0.9000000000000002,0.05994550408719346,0.05073020753266718,0.054954204829308906,1101,66,3488
+0.9500000000000003,0.06353591160220995,0.053036126056879324,0.057813154587348134,1086,69,3503
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