FSK modulator improvements

1. Phase is now changed proportional to the time of the bit change, if
   it happens somewhere between samples. The amount of phase change per
   bit is now the same for all bits, no matter how many samples
   currently lay inside this bit.

2. IIR filter is removed, because it causes distortions. A cosine shape
   is now used to transit between phase change of F0 to F1 and vice
   versa. This limits the spectrum. This filter is optional.

src/nmt/dsp.c

@@ -515,7 +515,7 @@ void nmt_set_dsp_mode(nmt_t *nmt, enum dsp_mode mode)
 {
 	/* reset frame */
 	if (mode == DSP_MODE_FRAME && nmt->dsp_mode != mode) {
-		fsk_mod_tx_reset(&nmt->fsk_mod);
+		fsk_mod_reset(&nmt->fsk_mod);
 		nmt->tx_frame_length = 0;
 	}