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/comb.h

@@ -30,9 +30,6 @@ public:
 	double get_snr();
 
 private:
-	gr_complex d_H[64];
-
-
 	const double alpha = 0.2;
 };