revisit particle system examples and use of "origin"

[shiffman]

the ParticleSystem classes having an enforced origin where particles begin is limiting and confusing to students.

[arastogi1111]

Pull Request #89 resolves this enhancement issue by adding mouse location as origination point for particles. If this is acceptable, I'll change for other examples too.
Let me know if this is not what you exactly wanted, I'll happily put enhancement.
Thank You for your excellent work. :)