v0.2.1 - Add parallel computing support

Release Note for RustyML v.0.2.1

Added

• Integrate Rayon for parallel computation across modules (7s faster in cargo test!!!)
• Add SVC(Support Vector Classification) support
• Add LinearSVC support
• Add KernelPCA support
• Add ProcessingError(String) to crate::ModelError
• Add LDA(Linear Discriminant Analysis) support
• Add t-Distributed Stochastic Neighbor Embedding (t-SNE) implementation
• Add variance calculation in math module
• Add MSE calculation in metric module
• Print info after training completes for fit functions of struct LDA, SVC, LinearSVC and PCA

Changed

• Refactor functions in math module and metric module to use ArrayView1 for improved efficiency
• Remove gaussian kernel calculation function, put gaussian kernel calculation directly in function fit of struct MeanShift
• Change the location of function standardize from crate::utility::principal_component_analysis to crate::utility

Removed

• Remove MSE calculation(named mean_squared_error, but actually calculate variance) in math module
• Remove gaussian kernel calculation function