The time_varying field output is two steps ahead of k-wave

[astanziola]

No description provided.

[btreeby]

This is solved as part of #33, in particular bc4accf. Time varying sources now have the same time axis as k-Wave. However, the output for p0 sources is still offset by one time step, as k-Wave adds the p0 at the end of time loop, and j-Wave at the beginning. We are documenting, but not modifying, this behaviour for now. See #35.