[Conversions] Add unitnr to IP address conversion

- Uses UDP Network member list to lookup the IP address
- Inspired by a [forum question](https://www.letscontrolit.com/forum/viewtopic.php?f=6&t=8066)

docs/source/Reference/SystemVariable.rst

@@ -189,3 +189,6 @@ The conversion always outputs a string, but not all of these can be converted ba
    * - To HEX: ``%c_2hex%(100000)``
      - To HEX: ``186A0``
      - Convert integer value to HEX notation.  (Added: 2020/10/07)
+   * - Unit to IP: ``%c_u2ip%(%unit%)``
+     - Unit to IP: ``192.168.1.67``
+     - Convert a (known) unit number to its IP Address.  (Added: 2020/11/08)

src/src/Helpers/Networking.cpp

@@ -334,14 +334,9 @@ void SendUDPCommand(byte destUnit, const char *data, byte dataLength)
 }
 
 /*********************************************************************************************\
-   Send UDP message (unit 255=broadcast)
+   Get IP address for unit
 \*********************************************************************************************/
-void sendUDP(byte unit, const byte *data, byte size)
-{
-  if (!NetworkConnected(10)) {
-    return;
-  }
-
+IPAddress getIPAddressForUnit(byte unit) {
   IPAddress remoteNodeIP;
 
   if (unit == 255) {
@@ -351,14 +346,30 @@ void sendUDP(byte unit, const byte *data, byte size)
     NodesMap::iterator it = Nodes.find(unit);
 
     if (it == Nodes.end()) {
-      return;
+      return remoteNodeIP;
     }
 
     if (it->second.ip[0] == 0) {
-      return;
+      return remoteNodeIP;
     }
     remoteNodeIP = it->second.ip;
   }
+  return remoteNodeIP;
+}
+
+/*********************************************************************************************\
+   Send UDP message (unit 255=broadcast)
+\*********************************************************************************************/
+void sendUDP(byte unit, const byte *data, byte size)
+{
+  if (!NetworkConnected(10)) {
+    return;
+  }
+
+  IPAddress remoteNodeIP = getIPAddressForUnit(unit);
+  if (remoteNodeIP[0] == 0) {
+    return;
+  }
 
 #ifndef BUILD_NO_DEBUG
 

src/src/Helpers/Networking.h

@@ -30,6 +30,11 @@ void checkUDP();
 \*********************************************************************************************/
 void SendUDPCommand(byte destUnit, const char *data, byte dataLength);
 
+/*********************************************************************************************\
+   Get IP address for unit
+\*********************************************************************************************/
+IPAddress getIPAddressForUnit(byte unit);
+
 /*********************************************************************************************\
    Send UDP message (unit 255=broadcast)
 \*********************************************************************************************/

src/src/Helpers/StringConverter.cpp

@@ -19,6 +19,7 @@
 #include "../Helpers/Convert.h"
 #include "../Helpers/ESPEasy_Storage.h"
 #include "../Helpers/Misc.h"
+#include "../Helpers/Networking.h"
 #include "../Helpers/Numerical.h"
 #include "../Helpers/StringParser.h"
 #include "../Helpers/SystemVariables.h"
@@ -790,6 +791,7 @@ void parseStandardConversions(String& s, boolean useURLencode) {
   SMART_CONV(F("%c_m2dhm%"),  minutesToDayHourMinute(arg1))
   SMART_CONV(F("%c_s2dhms%"), secondsToDayHourMinuteSecond(arg1))
   SMART_CONV(F("%c_2hex%"),   formatToHex(arg1, F("")))
+  SMART_CONV(F("%c_u2ip%"),   getIPAddressForUnit(arg1).toString())
   #undef SMART_CONV
 
   // Conversions with 2 parameters

src/src/WebServer/SysVarPage.cpp

@@ -200,6 +200,8 @@ void handle_sysvars() {
   addSysVar_html(F("Secs to dhms: %c_s2dhms%(100000)"));
   addFormSeparator(3);
   addSysVar_html(F("To HEX: %c_2hex%(100000)"));
+  addFormSeparator(3);
+  addSysVar_html(F("Unit to IP: %c_u2ip%(%unit%)"));
 
   html_end_table();
   html_end_form();