SPEAD, packet gap fixes

* SPEAD packet length was 2x too big.
* Gaps between correlator output products added to prevent flooding
of NIC input buffers/switch output buffers.
flags

Flags are carried through. If any flag goes high during an integration, all
products in the heap are flagged. Compiles but untested in hardware

pack_out blocks now use spead_pack blocks internally.

TODO:
* spead_pack block errors out with mutliple direct addressed items so only timestamp
carried through at moment, spead_pack fix still to be done.
* test in hardware
* check vacc for possible errors using vacc_tvg