Fix potential energy calculation.
Fix potential energy calculation.

Improve equality checking of partial derivatives.

jcrist/pydy,Shekharrajak/pydy,skidzo/pydy,Shekharrajak/pydy,jcrist/pydy,jcrist/pydy,skidzo/pydy,skidzo/pydy,oliverlee/pydy,jcrist/pydy,Shekharrajak/pydy,jcrist/pydy,oliverlee/pydy,oliverlee/pydy,jcrist/pydy,jcrist/pydy,Shekharrajak/pydy,skidzo/pydy