Refactor `BagExporter` to make it more readable

[hect95]

There are a few functions that can be broken down into smaller functions to improve readability.

For instance, BagExporter::extract_data(const fs::path & rosbag):

tartan_rosbag_exporter/ros2_bag_exporter/src/bag_exporter.cpp

Lines 118 to 128 in 0a00067

	void BagExporter::extract_data(const fs::path & rosbag)
	{
	// Initialize reader
	rosbag2_cpp::readers::SequentialReader reader;
	rosbag2_storage::StorageOptions storage_options;
	storage_options.uri = rosbag.string();
	storage_options.storage_id = storage_id_;
	rosbag2_cpp::ConverterOptions converter_options;
	converter_options.input_serialization_format = "cdr";
	converter_options.output_serialization_format = "cdr";