[WIP] Spearman correlation calculation

[Ta7ar]

Enables DataProfiler to calculate Spearman correlation with the options to allow users to enable/disable calculations or specify columns for calculations.

When ran on Iris dataset, the Pearson correlation and the Spearman correlation are very similar (as it should be when correlations are linear),
'correlation_matrix': array([[ 1. , -0.11756978, 0.87175378, 0.81794113, 0.78256123], [-0.11756978, 1. , -0.4284401 , -0.36612593, -0.42665756], [ 0.87175378, -0.4284401 , 1. , 0.96286543, 0.9490347 ], [ 0.81794113, -0.36612593, 0.96286543, 1. , 0.95654733], [ 0.78256123, -0.42665756, 0.9490347 , 0.95654733, 1. ]]), 'spearman_correlation_matrix': array([[ 1. , -0.16677766, 0.88189813, 0.83428878, 0.79807812], [-0.16677766, 1. , -0.30963509, -0.28903175, -0.44028958], [ 0.88189813, -0.30963509, 1. , 0.93766682, 0.93543052], [ 0.83428878, -0.28903175, 0.93766682, 1. , 0.93817917], [ 0.79807812, -0.44028958, 0.93543052, 0.93817917, 1. ]]),

Spearman mainly has 2 advantages over Pearson:

1. Robustness to outliers: When introduced an outlier in the 'petal length (cm)' column by iris['petal length (cm)'][random_index] = 100, Pearson correlation of the column changes drastically from,
   [ 0.87175378, -0.4284401 , 1. , 0.96286543, 0.9490347 ] to [ 0.23429883, -0.0503025 , 1. , 0.34474503, 0.30366187]
   Whereas Spearman correlation of the column changes from,
   [ 0.88189813, -0.30963509, 1. , 0.93766682, 0.93543052] to [ 0.88048828, -0.30861155, 1. , 0.9383322 , 0.93542968]
2. Detecting non-linear monotonic relationships: When introduced a new column as a exponential function of an old column by iris['exp'] = iris.apply(lambda row: row['petal length (cm)']**10, axis=1), the Pearson correlation between columns 'exp' and 'petal length (cm)' is 0.56452528 whereas Spearman correlation is 1.

Todo:

• Find out how/where to store ranked_mean and ranked_var for spearman correlation merging/updates
• Update ranked_mean and ranked_var on profile merging/updates
• Update Spearman correlation on profile merging/updates
• Write tests