Mass should be bigger than zero

[imtrobin]

Hi

Just like to report, the mass pass in should be bigger than zero, and I think there should be an assertion to catch these invalid values. When I pass in 0 for mass, it creates but somewhere later, there is a Nan exception.

[Norbo]

Thanks; I plan to add in some additional input protection in various functions and around the update method to catch common errors and provide suggestions in the upcoming version.