Added setDescription(const char *) method to SpanCharacteristic

This allows you to add an optional "description" to a Characteristic.  This is not the same as the name of a Service and is generally not used by the Home App.  However, it appears to be consumed by the Eve app, which can be helpful in some circumstances when developing custom characteristics.

src/HomeSpan.h

@@ -601,6 +601,12 @@ struct SpanCharacteristic{
     return(setPerms(perms&(~dPerms)));
   }
 
+  SpanCharacteristic *setDescription(const char *c){
+    desc = (char *)realloc(desc, strlen(c) + 1);
+    strcpy(desc, c);
+    return(this);
+  }  
+
 };
 
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