Make d_H inherited from base class.

All equalizer classes had their own private `d_H` vector, despite all being derived from a common `base` class. Now `d_H` is instead a protected member of `base`, and thus accessible to the derived equalizer classes.

lib/equalizer/sta.h

@@ -31,7 +31,6 @@ public:
 	double get_snr();
 
 private:
-	gr_complex d_H[64];
 	double d_snr;
 
 	const double alpha = 0.5;