Transceiver52M: Add dual channel diversity receiver option

This patch add support for dual channel diversity on the receive
path. This allows two antennas two shared antennas to be used for
each ARFCN handling channel in the receiver. This configuration
may improvde performance in multi-path fading environments,
however, noise andpotential interference levels are increased due
to the higher bandwidth used.

The receive path is oversampled by a factor of four for a rate
of 1.083333 Msps. If the receive paths are tuned within a
maximum channel spacing (currently set at 600 kHz), then both
ARFCN frequencies are processed by each channel of the receiver.
Otherwise, the frequency shifted diversity path is disabled and
standard non-diversity operation takes place.

Diversity processing is handled by selecting the path with the
higheset energy level and discarding the burst on the second
path. Selection occurs on a burst-by-burst basis.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Check time slot validity of incoming bursts

In errant cases, GSM core may send bursts with invalid slot values,
which is allowed by the GSM::Time object. If we find a burst like this
coming into the transceiver, then drop it immediately.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Ignore detected bursts at the noise floor

The transceiver has the ability to detect bursts below the noise floor,
but little hope in successful decoding, so don't even try. We still use
the detected burst to differentiate against noise vs actual data.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Use independent noise vectors for each channel

Each ARFCN channel may be independently configureted and possibly on
separate hardware, so don't share a single vector for noise estimate
calculations. Allow a non-pointer based iterator so we can get away
with using the default copy constructor.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Set const qualifier on appropriate radio vector methods

Pointer accessor and noise average methods for radio and noise vectors
respectively.

Signed-off-by: Thomas Tsou <tom@tsou.cc>