Particle
========

A particle is a mass that is fixed to a point, with no inertia tensor.

**Keyword Arguments**

:param str name: Key to be used when referencing this particle.

:param str ref_point_key: Key of the point to which this particle is fixed.

:param str mass: Mass to be assigned to this particle, usually an existing parameter (symbolic constant).

:return str: Message describing the particle.