clear particles when switching domains
Merge pull request #1687 from ZappoMan/particles_switch_domains

clear particles when switching domains
added findParticles() to JS api
added Particles.getParticleProperties() to JS
add example
fix comment
handle preventing edits to non-existant particles
added new example of particle bird
Merge pull request #1694 from ZappoMan/dont_edit_bad_particles

New particles JS features