Merge pull request #15 from agbeltran/dev

Changes related to the definition of ratio
Merge pull request #17 from agbeltran/dev

Changes related to 'statistical hypothesis testing'
Merge pull request #19 from agbeltran/dev

Changed label
adding covariance, revising axioms on correlation coefficient accordingly
adding two-sample t-test, changing class label for two-samples Hotelling t-test to two-sample Hotelling t-test
adding two-sample t-test assuming unequal varirance aka Welch test
Catalog file - committed only once