Shrink sensors code size by 5k using local pow

lib/Mutichannel_Gas_Sensor/src/MutichannelGasSensor.cpp

@@ -327,6 +327,30 @@ int16_t MutichannelGasSensor::readR(void)
 ** Returns:
                             float value - concentration of the gas
 *********************************************************************************************************/
+float MutichannelGasSensor_pow(float a, float b)
+{
+  // https://martin.ankerl.com/2012/01/25/optimized-approximative-pow-in-c-and-cpp/
+  // calculate approximation with fraction of the exponent
+  int e = abs((int)b);
+  union {
+    double d;
+    int x[2];
+  } u = { a };
+  u.x[1] = (int)((b - e) * (u.x[1] - 1072632447) + 1072632447);
+  u.x[0] = 0;
+  // exponentiation by squaring with the exponent's integer part
+  // double r = u.d makes everything much slower, not sure why
+  double r = 1.0;
+  while (e) {
+    if (e & 1) {
+      r *= a;
+    }
+    a *= a;
+    e >>= 1;
+  }
+  return r * u.d;
+}
+
 float MutichannelGasSensor::calcGas(int gas)
 {
 
@@ -370,42 +394,42 @@ float MutichannelGasSensor::calcGas(int gas)
     {
         case CO:
         {
-            c = pow(ratio1, -1.179)*4.385;  //mod by jack
+            c = MutichannelGasSensor_pow(ratio1, -1.179)*4.385;  //mod by jack
             break;
         }
         case NO2:
         {
-            c = pow(ratio2, 1.007)/6.855;  //mod by jack
+            c = MutichannelGasSensor_pow(ratio2, 1.007)/6.855;  //mod by jack
             break;
         }
         case NH3:
         {
-            c = pow(ratio0, -1.67)/1.47;  //modi by jack
+            c = MutichannelGasSensor_pow(ratio0, -1.67)/1.47;  //modi by jack
             break;
         }
         case C3H8:  //add by jack
         {
-            c = pow(ratio0, -2.518)*570.164;
+            c = MutichannelGasSensor_pow(ratio0, -2.518)*570.164;
             break;
         }
         case C4H10:  //add by jack
         {
-            c = pow(ratio0, -2.138)*398.107;
+            c = MutichannelGasSensor_pow(ratio0, -2.138)*398.107;
             break;
         }
         case GAS_CH4:  //add by jack
         {
-            c = pow(ratio1, -4.363)*630.957;
+            c = MutichannelGasSensor_pow(ratio1, -4.363)*630.957;
             break;
         }
         case H2:  //add by jack
         {
-            c = pow(ratio1, -1.8)*0.73;
+            c = MutichannelGasSensor_pow(ratio1, -1.8)*0.73;
             break;
         }
         case C2H5OH:  //add by jack
         {
-            c = pow(ratio1, -1.552)*1.622;
+            c = MutichannelGasSensor_pow(ratio1, -1.552)*1.622;
             break;
         }
         default: