Enhance network generation capabilities with string parsing and connection management

[CardamaS99]

This pull request introduces a new feature for generating a network from a string representation and manage connections between hosts in the QuNetSim library. This module simplifies the process of setting up complex quantum and classical network topologies.

Key Features

• Network Generation: Create networks using a simple string representation.
• Connection Management: Automatically handle classical, quantum, and combined connections between hosts.
• Regular Expression Parsing: Efficiently parse and interpret network configurations.

Syntax

• Alice <==> Bob: to represent a classical and quantum connection.
• Alice <~~> Bob: to represent a quantum only connection.
• Alice <--> Bob: to represent a classical only connection.

All connections are added uni-directionally, so < and > represent the direction of the flow of traffic.

Documentation and Examples:

• README.md: Added a new quick example demonstrating how to generate a network from a string.
• examples/network_generated/send_qubit.py: Added a new example script showcasing the usage of the network generation feature.

New Modules and Functions:

• qunetsim/__init__.py: Updated to import the new generator module.
• qunetsim/generator/__init__.py: Added a docstring and imported the network_generate function.
• qunetsim/generator/conn_type.py: Introduced ConnType class to handle different connection types.
• qunetsim/generator/connection.py: Added Connection class to represent connections between hosts.
• qunetsim/generator/direction.py: Added Direction class to handle connection directions.
• qunetsim/generator/generate.py: Implemented functions to parse network strings and generate networks.