Fix sorting of f args in PE function.
Fix sorting of f args in PE function.

Fix sorting of arguments of f functions when calculating a potential
energy function of a system.

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