gsr: Added gsr sensor C source

Added the C source for the gsr sensor with necessary changes to cmake,
examples, docs.

    * Renamed all files with grovegsr to gsr
    * Replaced all instances of grovegsr with gsr
    * Added C source for gsr sensor
    * Updated all cmake files
    * Added C example for gsr sensor
    * Fixed C++ threshold method from throwing on a successfull aio read

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/gsr/gsr.h

@@ -0,0 +1,133 @@
+/*
+ * Author:
+ * Copyright (c) 2015 Intel Corporation.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#pragma once
+
+#include "upm.h"
+#include "mraa/aio.h"
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * driver context
+ */
+typedef struct _gsr_context {
+    /* mraa aio pin context */
+    mraa_aio_context aio;
+    /* Analog voltage reference */
+    float m_aRef;
+    /* Scale */
+    float m_scale;
+    /* Offset in sensor units */
+    float m_offset;
+} *gsr_context;
+
+/**
+ * Initialize analog sensor
+ * @param pin Analog pin
+ * @return sensor context
+ */
+gsr_context gsr_init(int16_t pin);
+
+/**
+ * Analog sensor destructor
+ * @param sensor context pointer
+ */
+void gsr_close(gsr_context dev);
+
+/**
+ * Set ADC reference voltage
+ * @param dev sensor context pointer
+ * @param aref ADC reference voltage
+ * @return Function result code
+ */
+upm_result_t gsr_set_aref(const gsr_context dev, float aref);
+
+/**
+ * Set sensor scale.  This scale is applied to the return value:
+ *     counts = counts * scale
+ * @param dev sensor context pointer
+ * @param scale count scale value used
+ * @return Function result code
+ */
+upm_result_t gsr_set_scale(const gsr_context dev, float scale);
+
+/**
+ * Set sensor offset.  This offset is applied to the return value:
+ *     value = value + offset
+ * @param dev sensor context pointer
+ * @param offset count offset value used
+ * @return Function result code
+ */
+upm_result_t gsr_set_offset(const gsr_context dev, float offset);
+
+/**
+ * Get sensor aref
+ * @param dev sensor context pointer
+ * @return Sensor ADC reference voltage
+ */
+float gsr_get_aref(const gsr_context dev);
+
+/**
+ * Get sensor scale
+ * @param dev sensor context pointer
+ * @return Sensor scale
+ */
+float gsr_get_scale(const gsr_context dev);
+
+/**
+ * Get sensor offset
+ * @param dev sensor context pointer
+ * @return Sensor offset
+ */
+float gsr_get_offset(const gsr_context dev);
+
+/**
+ * Read normalized value for sensor
+ * @param dev sensor context pointer
+ * @param *value Normalized value (0.0 -> 1.0)
+ * @return Function result code
+ */
+upm_result_t gsr_get_normalized(const gsr_context dev, float *value);
+
+/**
+ * Read raw voltage from the sensor
+ * @param dev sensor context pointer
+ * @param *value Raw sensor voltage
+ * @return Function result code
+ */
+upm_result_t gsr_get_raw_volts(const gsr_context dev, float *value);
+
+/**
+ * Read scaled/offset voltage from the sensor
+ * @param dev sensor context pointer
+ * @param *value Adjusted sensor voltage
+ * @return Function result code
+ */
+upm_result_t gsr_get_volts(const gsr_context dev, float *value);
+#ifdef __cplusplus
+}
+#endif