feat: Add machine learning models and data struct to vsl.ml

ml/README.md

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+# VSL Machine Learning (vsl.ml)
+
+VSL aims to provide a robust set of tools for scientific computing with an emphasis
+on performance and ease of use. In the `vsl.ml` module, some machine learning 
+models are designed as observers of data, meaning they re-train automatically when 
+data changes, while others do not require this functionality.
+
+## Key Features
+
+- **Observers of Data**: Some machine learning models in VSL act as observers,
+re-training automatically when data changes.
+- **High Performance**: Leverages V’s performance optimizations and can integrate
+with C and Fortran libraries like Open BLAS and LAPACK.
+- **Versatile Algorithms**: Supports a variety of machine learning algorithms and
+models.
+
+## Usage
+
+### Loading Data
+
+The `Data` struct in `vsl.ml` is designed to hold data in matrix format for machine
+learning tasks. Here's a brief overview of how to use it:
+
+#### Creating a Data Object
+
+You can create a `Data` object using the following methods:
+
+- `Data.new`: Creates a new `Data` object with specified dimensions.
+- `Data.from_raw_x`: Creates a `Data` object from raw x values (without y values).
+- `Data.from_raw_xy`: Creates a `Data` object from raw x and y values combined in a single matrix.
+- `Data.from_raw_xy_sep`: Creates a `Data` object from separate x and y raw values.
+
+### Data Methods
+
+The `Data` struct has several key methods to manage and manipulate data:
+
+- `set(x, y)`: Sets the x matrix and y vector and notifies observers.
+- `set_y(y)`: Sets the y vector and notifies observers.
+- `set_x(x)`: Sets the x matrix and notifies observers.
+- `split(ratio)`: Splits the data into two parts based on the given ratio.
+- `clone()`: Returns a deep copy of the Data object without observers.
+- `clone_with_same_x()`: Returns a deep copy of the Data object but shares the same x reference.
+- `add_observer(obs)`: Adds an observer to the data object.
+- `notify_update()`: Notifies observers of data changes.
+
+### Stat Observer
+
+The `Stat` struct is an observer of `Data`, providing statistical analysis of the
+data it observes. It automatically updates its statistics when the underlying data
+changes.
+
+## Observer Models
+
+The following machine learning models in VSL are compatible with the `Observer`
+pattern. This means they can observe data changes and automatically update
+themselves.
+
+### K-Means Clustering
+
+K-Means Clustering is used for unsupervised learning to group data points into
+clusters. As an observer model, it re-trains automatically when the data changes,
+which is useful for dynamic datasets that require continuous updates.
+
+### K-Nearest Neighbors (KNN)
+
+K-Nearest Neighbors (KNN) is used for classification tasks where the target
+variable is categorical. As an observer model, it re-trains automatically when the
+data changes, which is beneficial for datasets that are frequently updated.
+
+## Non-Observer Models
+
+The following machine learning models in VSL do not require the observer pattern
+and are trained once on a dataset without continuous updates.
+
+### Linear Regression
+
+Linear Regression is used for predicting a continuous target variable based on one
+or more predictor variables. It is typically trained once on a dataset and used to
+make predictions without requiring continuous updates. Hence, it is not implemented
+as an observer model.