Adjust reset flag interpolation to trigger at jump

[pvillacorta]

Testing with FlowPath, I realized that the interpolation of the reset flag is performed using the Constant{Previous} option. This required the spin_reset matrix to be true one time step before the jump, which doesn't make much sense.

I believe it’s better for spin_reset to activate at the time node where the jump occurs, rather than the one before it. This approach simplifies phantom design and seems more intuitive.

For a single spin, where the jump occurs when the spin exits at the top (dz = 4) and reenters at the bottom (dz = -4):

Previously, the phantom had to be designed like this:

dz         = [1 2 3 4 -4 -3 -1 -1]  
spin_reset = [0 0 0 1  0  0  0  0]  

Now:

dz         = [1 2 3 4 -4 -3 -1 -1]  
spin_reset = [0 0 0 0  1  0  0  0]  

So, this PR only changes Previous to Next in outflow_spin_reset!.