Add support for particle counts, backwards compatible with PMSx003

[kintel]

This is a rewrite of #7 which should fix #8 in a better way:

• Checks frame length and only populates the particle fields when large frames are received
• Adds a boolean indicating if the particles fields were populated
• Modifies the Advanced example to show how to use this

Tested on PMS5003 in both active and passive mode.