Simple[NetDevice,Channel] - not that simple anymore

Simple[NetDevice,Channel] - not that simple anymore

[Tom Henderson, 2014-09-05 20:23:27]

I think this is ready to merge.  I might add that we should improve the doxygen
which is stale:

 * This device does not have a helper and assumes 48-bit mac addressing;
 * the default address assigned to each device is zero, so you must
 * assign a real address to use it.  There is also the possibility to
 * add an ErrorModel if you want to force losses on the device.

and we should also caution users that all receivers on a broadcast channel will
likely receive the packet at exactly the same time, and no interference
(collision) is modeled.

https://codereview.appspot.com/132870043/diff/40001/src/network/utils/simple-...
File src/network/utils/simple-net-device.cc (right):

https://codereview.appspot.com/132870043/diff/40001/src/network/utils/simple-...
src/network/utils/simple-net-device.cc:421: if (m_bps > DataRate ())
for clarity, how about 
m_bps > DataRate (0)

(also below)

https://codereview.appspot.com/132870043/diff/40001/src/network/utils/simple-...
src/network/utils/simple-net-device.cc:423: txTime = Seconds
(m_bps.CalculateTxTime (packet->GetSize ()));
I was surprised to see that CalculateTxTime returns a double not a Time--
perhaps we need to (separately) improve that.