Add helper function to convert from ICRS to galactic coords

Also includes creating a util file for coordinate conversions

src/util/coordinateconversion.cpp

@@ -0,0 +1,49 @@
+/*****************************************************************************************
+ *                                                                                       *
+ * OpenSpace                                                                             *
+ *                                                                                       *
+ * Copyright (c) 2014-2020                                                               *
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+ ****************************************************************************************/
+
+#include <openspace/util/coordinateconversion.h>
+
+namespace openspace {
+
+glm::dvec3 icrsToGalacticCartesian(float ra, float dec, double distance) {
+    // Convert to Galactic Coordinates from ICRS right ascension and declination
+    // https://gea.esac.esa.int/archive/documentation/GDR2/Data_processing/
+    // chap_cu3ast/sec_cu3ast_intro/ssec_cu3ast_intro_tansforms.html#SSS1
+    const glm::dmat3 conversionMatrix = glm::dmat3({
+        -0.0548755604162154,  0.4941094278755837, -0.8676661490190047, // col 0
+        -0.8734370902348850, -0.4448296299600112, -0.1980763734312015, // col 1
+        -0.4838350155487132,  0.7469822444972189,  0.4559837761750669  // col 2
+    });
+
+    glm::dvec3 rICRS = glm::dvec3(
+        cos(glm::radians(ra)) * cos(glm::radians(dec)),
+        sin(glm::radians(ra)) * cos(glm::radians(dec)),
+        sin(glm::radians(dec))
+    );
+    glm::dvec3 rGalactic = conversionMatrix * rICRS; // on the unit sphere
+
+    return distance * rGalactic;
+}
+
+} // namespace openspace