Filter improvement: LP and HP filters, added test function

The -3 dB level at cut-off frequency is now maintained for multiple
iterations.

src/cnetz/scrambler.h

@@ -1,9 +1,9 @@
 #include "../common/filter.h"
 
 typedef struct scrambler {
-	double			carrier_phaseshift256;	/* carrier phase shift per sample */
-	double			carrier_phase256;	/* current phase of carrier frequency */
-	filter_lowpass_t	lp;			/* filter to remove carrier frequency */
+	double		carrier_phaseshift256;	/* carrier phase shift per sample */
+	double		carrier_phase256;	/* current phase of carrier frequency */
+	filter_t	lp;			/* filter to remove carrier frequency */
 } scrambler_t;
 
 void scrambler_init(void);