Add PAMmapper bit mapping and Gray penalty visualization

Theory/Mapping_Coding/Gray_penalty.m

@@ -0,0 +1,11 @@
+function [Gp, details] = Gray_penalty(use_eth_mapping)
+%GRAY_PENALTY Average bit errors per nearest-neighbor PAM-6 symbol error.
+
+if nargin < 1
+    use_eth_mapping = 1;
+end
+
+mapper = PAMmapper(6, 0, "eth_style", use_eth_mapping);
+mapper.plotBitMapping();
+[Gp, details] = mapper.graypenalty();
+end