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>

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;