Add sfm3x00 example

* Add minimal example for SFM3x00 familiy

* clarify MODEL selection enum

* add README describing Model configuration

Author: winkj

SFM3x00_I2C_minimal_example/README.md

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+# SFM3x00 Minimal Example
+
+## Configuration
+
+Please note that the model selection is done manually, line 55 of [SFM3x00_minimal_example.ino](SFM3x00_minimal_example.ino#55)
+
+```c++
+54: // ACTION: select your component here from the enum above:
+55: const uint8_t MODEL = SFM3200;
+```
+
+The available sensors can be found on line 36 of [SFM3x00_minimal_example.ino](SFM3x00_minimal_example.ino#36)
+
+```c++
+35: // supported sensors
+36: enum SFM_MODEL {
+37:   SFM3000 = 0,
+38:   SFM3200,
+39:   SFM3300,
+40:   SFM3400,
+41:   SFM_MODEL_LENGTH //< Note: this is not a valid value for 'MODEL' below
+42: };
+```

SFM3x00_I2C_minimal_example/SFM3x00_minimal_example.ino

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+/*
+ * Copyright (c) 2021, Sensirion AG
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, this
+ *   list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ *
+ * * Neither the name of Sensirion AG nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+
+#include <Wire.h>
+
+// supported sensors
+enum SFM_MODEL {
+  SFM3000 = 0,
+  SFM3200,
+  SFM3300,
+  SFM3400,
+  SFM_MODEL_LENGTH //< Note: this is not a valid value for 'MODEL' below
+};
+
+// from the datasheets:
+const int16_t SFM3X00_ADDRESS                    =  0x40;
+
+const int16_t FLOW_OFFSET[SFM_MODEL_LENGTH]      = { 32000, 32768, 32768, 32768 };
+const int16_t SCALE_FACTOR_AIR[SFM_MODEL_LENGTH] = {   140,   140,   120,   800 };
+
+const byte    CMD_START_MEASUREMENT[]            = { 0x10, 0x00 }; 
+const byte    CMD_SOFT_RESET[]                   = { 0x20, 0x00 }; 
+
+
+// ACTION: select your component here from the enum above:
+const uint8_t MODEL = SFM3200;
+
+void setup() 
+{
+  Serial.begin(115200);
+  while(!Serial);
+  
+  Wire.begin();
+
+  /*
+  // optional reset before use
+  Wire.beginTransmission(SFM3X00_ADDRESS);
+  Wire.write(CMD_SOFT_RESET, 2);
+  Wire.endTransmission();
+  delay(100);
+  */
+
+  Wire.beginTransmission(SFM3X00_ADDRESS);
+  Wire.write(CMD_START_MEASUREMENT, 2);
+  Wire.endTransmission();
+
+  delay(100);
+  Serial.println("Flow rate [slm]");
+}
+
+void loop() 
+{
+  Wire.requestFrom(SFM3X00_ADDRESS, 3);
+  if (Wire.available() < 3) {
+    Serial.println("No data received from sensor");
+  } else {
+    int16_t flow;
+    flow  = (int16_t)Wire.read() << 8;
+    flow |= Wire.read();
+    // CRC verification (third byte) left as an exercise for the reader
+
+    flow = (flow - FLOW_OFFSET[MODEL]) / SCALE_FACTOR_AIR[MODEL];
+    Serial.println(flow);
+  }
+
+  delay(100);
+}