Add filter option to read n chunks or bytes about rosbag.js

[jtbandes]

It's actually possible to use the BagReader class directly (rather than Bag) for more low-level access:

rosbag.js/src/BagReader.js

Lines 33 to 37 in 96ccb2a

	// BagReader is a lower level interface for reading specific sections & chunks
	// from a rosbag file - generally it is consumed through the Bag class, but
	// can be useful to use directly for efficiently accessing raw pieces from
	// within the bag
	export default class BagReader {

You can see how the Bag class uses it to implement the higher-level functionality:

rosbag.js/src/bag.js

Lines 54 to 58 in 96ccb2a

	async open() {
	this.header = await this.reader.readHeaderAsync();
	const { connectionCount, chunkCount, indexPosition } = this.header;
	const result = await this.reader.readConnectionsAndChunkInfoAsync(indexPosition, connectionCount, chunkCount);

[foobarbecue]

Wonderful, thanks!