Add nice data access API

[ThomasDebrunner]

This changes the library the following way:

• The field class now also stores sizes and offsets in the message format
• The value class is overhauled to automatically cast to the right types based on the message definitions
• The value class can automatically cast to the users desired types
• The MessageFormat contains all information needed to parse the message
• The MessageFormats are resolved recursively once the header is received

Still to do:

• Nested types are resolved correctly, but the API to access in a nested way is missing
• Make sure all the previous tests and use cases are still supported
• Create new tests for the added functionlity

This allows the user to read data like this:

  // Read out some data

  // List all subscription names
  for (const auto& sub : data_container->subscriptionNames()) {
    std::cout << sub << std::endl;
  }

  // Get a particular subscription
  const auto& subscription = data_container->subscription("vehicle_status");

  // Get message format of subscription
  auto message_format = subscription->format();
  std::cout << "Message format: " << message_format->name() << std::endl;

  // List all field names
  for (const std::string &field : subscription->fieldNames()) {
    std::cout << field << std::endl;
  }

  // Get particular field
  auto nav_state_field = subscription->field("nav_state");

  // Iterate over all samples
  for (const auto &sample : *subscription) {
    // always correctly extracts the type as defined in the message definition,
    // gets cast to the value you put in int.
    // This also works for arrays and strings.
    auto nav_state = sample[nav_state_field].as<int>();
    std::cout << nav_state << std::endl;
  }

  // get a specific sample
  auto sample_12 = subscription->at(12);

  // access values by name
  auto timestamp = sample_12["timestamp"].as<uint64_t>();

  std::cout << timestamp << std::endl;

[bkueng]

Very cool! I like the type magic.

Would be good to have some unit tests for the nested data access case.

[ThomasDebrunner]

Thank you for the review! I'll work on adding more unit tests.

[ThomasDebrunner]

Also merged in main

[ThomasDebrunner]

Also added a bunch of unit testing for the added functionality