Calculate velocities for trajectories

[ghutchis]

We can estimate numeric velocities from the differences in coordinate positions. Since the timestep must be constant, we should take these deltas and estimate the velocities on each atom.

[harikrishna-au]

I’ve implemented an estimateVelocities() method in the Molecule class to compute velocities from coordinate differences.
It applies a forward difference for the first frame and a backward difference for all remaining frames so every timestep has a value.Results are stored in a new m_velocities member and accessed via velocities(index).Before I open the PR, does this approach align with expectations?