Fix polyphase aligned with `mode="rate"`

[mhostetter]

We should not prepend B-1 zeros when using rate mode. This is so that the signal perfectly aligns with the decimation.

This is the implementation for non-streaming. Need to think more carefully about streaming mode and its state.

    else:
        # Prepend zeros to so the first sample is alone in the first commutated column
        if mode == "full":
            print("doing x_pad for full")
            x_pad = np.insert(x, 0, np.zeros(B - 1))
        else:
            print("skipping x_pad for rate")
            x_pad = x
        # x_pad = np.insert(x, 0, np.zeros(B - 1))