Transceiver52M: Dynamically allocate convolution input vectors

This prevents the use of a copy constructor in the downlink
modulator and prevents a secondary memory allocation during
the convolution. Avoid both cases by dynamically allocating
with preloaded head room. The latter provides enough memory
before the first sample in the burst to cover the length
of the filter taps.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
diff --git a/Transceiver52M/sigProcLib.h b/Transceiver52M/sigProcLib.h
index 516d352..ebe7519 100644
--- a/Transceiver52M/sigProcLib.h
+++ b/Transceiver52M/sigProcLib.h
@@ -95,6 +95,9 @@
       symmetry = NONE;
     };
 
+  /** start index */
+  int getStartIndex() const { return mStart - mData; };
+
   /** symmetry operators */
   Symmetry getSymmetry() const { return symmetry;};
   void setSymmetry(Symmetry wSymmetry) { symmetry = wSymmetry;};