Add functionality of Partial VHF

[rmatsum836]

For a partial VHF, we are considering a binary system of A + B. There are two cases in which:

• particle A is at t=0 and r=0, and particle B is at r=r at time t
• particle B is at t=0 and r=0, and particle A is at r=r at time t

The idea is to edit the current function such that these cases can be calculated separately from one another.

[rmatsum836]

Closing this issue as this is already handled. This just requires the user intelligently pass the pairs and time tuples for each case.