Hide pin color string + box if color is unknown

Removes requirement of using `__` to show pins without color. Any non-color string can be used: `-`, `--`, `__`, `X`, `""` are some options.

src/wireviz/Harness.py

@@ -114,7 +114,7 @@ def create_graph(self) -> Graph:
                                         f'<td>{pinlabel}</td>' if pinlabel else None])
 
                     if connector.pincolors:
-                        if not connector.pincolors[counter] == "__":
+                        if connector.pincolors[counter] in wv_colors._color_hex.keys():
                             # the cell with the pincolor string and
                             # the cell with the color box
                             # need to be separate, since GraphViz