Improve calculation of partial velocities.
Improve calculation of partial velocities.

Express velocity in desired frame before substitutions when calculating
partial velocities.

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