transceiver: add Laurent C1 pulse shape to modulator

Remove existing GMSK filter approximation and replace with Laurent
decomposion based on primary and second pulses. Working case for
4 samples-per-symbol only. Start and end of burst bits are not
fully handled and may produce phase and magnitude deviations.

Add Laurent C1 pulse to modulator based on L = 3 pulse series. The
full expansion for L = 3 containes pulses C0 through C3. Pulses C2
and C3 are neglible and we ignore them completely.

Filter coefficients are generated separately from Octave/Matlab.
The '.m' file is included.

Pierre Laurent, "Exact and Approximate Construction of Digital Phase
    Modulations by Superposition of Amplitude Modulated Pulses", IEEE
    Transactions of Communications, Vol. 34, No. 2, Feb 1986.

Signed-off-by: Thomas Tsou <ttsou@vt.edu>
transceiver: minimize start/end of burst phase transients

In order to minimize excessive phase errors at the beginning
and end of bursts, add start/end bits for differential
encoding, which avoids the phase transients. Also zero and
shape the ramp up amplitude.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
transceiver52m: use single pulse shape for receive correlation

We're not concerned about absolute GMSK phase accuracy on the
receive side. Leave out the C1 pulse unless it can be shown that
the added complexity actually improves receive performance.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
umtrx: setup for 4 samples per symbol and set offset timing

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