Added power off to common

And implemented for ublox

src/TinyGsmClientBG96.h

@@ -308,8 +308,9 @@ public:
   }
 
   bool poweroff() {
-    sendAT(GF("+QPOWD"));
-    return waitResponse(GF("POWERED DOWN")) == 1; // TODO
+    sendAT(GF("+QPOWD=1"));
+    waitResponse(300);  // returns OK first
+    return waitResponse(300, GF("POWERED DOWN")) == 1;
   }
 
   bool radioOff() {

src/TinyGsmClientESP8266.h

@@ -297,7 +297,10 @@ public:
     return init();
   }
 
-  bool poweroff() TINY_GSM_ATTR_NOT_IMPLEMENTED;
+  bool poweroff() {
+    sendAT(GF("+GSLP=0"));  // Power down indefinitely - until manually reset!
+    return waitResponse() == 1;
+  }
 
   bool radioOff() TINY_GSM_ATTR_NOT_IMPLEMENTED;
 

src/TinyGsmClientM95.h

@@ -316,8 +316,8 @@ public:
   }
 
   bool poweroff() {
-    sendAT(GF("+QPOWD"));
-    return waitResponse(GF("POWERED DOWN")) == 1; // TODO
+    sendAT(GF("+QPOWD=1"));
+    return waitResponse(300, GF("NORMAL POWER DOWN")) == 1;
   }
 
   bool radioOff() {

src/TinyGsmClientUBLOX.h

@@ -325,7 +325,10 @@ public:
     return init();
   }
 
-  bool poweroff() TINY_GSM_ATTR_NOT_IMPLEMENTED;
+  bool poweroff() {
+    sendAT(GF("+CPWROFF"));
+    return waitResponse(40000L) == 1;
+  }
 
   bool radioOff() {
     sendAT(GF("+CFUN=0"));

src/TinyGsmClientXBee.h

@@ -402,7 +402,9 @@ public:
     exitCommand();
   }
 
-  bool poweroff() TINY_GSM_ATTR_NOT_IMPLEMENTED;
+  bool poweroff() {  // Not supported
+    return false;
+  }
 
   bool radioOff() TINY_GSM_ATTR_NOT_IMPLEMENTED;
 

src/TinyGsmCommon.h

@@ -239,6 +239,7 @@ public:
    */
 
   virtual bool restart() = 0;
+  virtual bool poweroff() = 0;
 
   /*
    * SIM card functions - only apply to cellular modems
@@ -282,6 +283,7 @@ public:
     return false;
   }
   virtual bool gprsDisconnect() { return false; }
+  virtual bool isGprsConnected() { return false; }
 
   /*
    * IP Address functions