Translational Spring
====================

A translational spring/damper induces an attractive force between two points that is colinear with a vector drawn from the start to end point. The force magnitude is

    F = k (x - e) + c x'
    
where k is the spring stiffness, x is the distance between the two points, e is an equilibrium length of the spring, c is the damping constant, and x' is the differential speed between the two points.

**Keyword Arguments**

:param str start_point: Key for the first point.

:param str end_point: Key for the second point.

:param k: Parameter corresponding to the spring stiffness constant.
:type k: str or numeric

:param c: Parameter corresponding to the damping constant.
:type c: str or numeric

:param equilibrium: Parameter corresponding to the equilibrium length of the spring.
:type equilibrium: str or numeric

:return str: Message describing the created spring