Improve extract features and doc

Module API changes

• Make extract_features a public function
• Remove the features parameter from machine_learn, deep_learn and deep_transfer_learn
• Make path a required argument for the load function
• Improve FileNotFoundError message for dataset loading
• Add imports to the __init__.py
• Improve package loading speed
• Enable to print package version number
• Fix legacy LinearSVC in machine_learn

Main script changes

• Move all main scripts into an examples folder
• Merge the two versions of transfer learning (with and without unlabelled segments)

Doc

• Add main scripts as examples to the doc
• Update simple use case example
• Add installation instructions for development

Other changes

• Simplify the repository structure by removing the src folder
• Add data file extensions to the gitignore
• Version number