Implementation of Standard scaler (#143)

* docs: Fix typo in doc for categorical transformer.
* feat: Add option to take a column from Matrix.
I created the method `Matrix::take_column` that uses the `Matrix::take`-interface to extract a single column from a matrix. I need that feature in the implementation of  `StandardScaler`.
* feat: Add `StandardScaler`.
Authored-by: titoeb <timtoebrock@googlemail.com>

Author: titoeb

src/linalg/mod.rs

@@ -651,6 +651,10 @@ pub trait BaseMatrix<T: RealNumber>: Clone + Debug {
 
         result
     }
+    /// Take an individual column from the matrix.
+    fn take_column(&self, column_index: usize) -> Self {
+        self.take(&[column_index], 1)
+    }
 }
 
 /// Generic matrix with additional mixins like various factorization methods.
@@ -761,4 +765,21 @@ mod tests {
         assert_eq!(m.take(&vec!(1, 1, 3), 0), expected_0);
         assert_eq!(m.take(&vec!(1, 0), 1), expected_1);
     }
+
+    #[test]
+    fn take_second_column_from_matrix() {
+        let four_columns: DenseMatrix<f64> = DenseMatrix::from_2d_array(&[
+            &[0.0, 1.0, 2.0, 3.0],
+            &[0.0, 1.0, 2.0, 3.0],
+            &[0.0, 1.0, 2.0, 3.0],
+            &[0.0, 1.0, 2.0, 3.0],
+        ]);
+
+        let second_column = four_columns.take_column(1);
+        assert_eq!(
+            second_column,
+            DenseMatrix::from_2d_array(&[&[1.0], &[1.0], &[1.0], &[1.0]]),
+            "The second column was not extracted correctly"
+        );
+    }
 }

src/preprocessing/mod.rs

@@ -1,5 +1,7 @@
-/// Transform a data matrix by replaceing all categorical variables with their one-hot vector equivalents
+/// Transform a data matrix by replacing all categorical variables with their one-hot vector equivalents
 pub mod categorical;
 mod data_traits;
+/// Preprocess numerical matrices.
+pub mod numerical;
 /// Encode a series (column, array) of categorical variables as one-hot vectors
 pub mod series_encoder;