Feature/volume tools (#614)

* Create volume generation task

* Change domain bounds into a non-optional parameter

* Work on volume generation and rendering

* Example generation of time varying volume

* Example generation of time varying volume

* Compile fix in RenderableKameleonVolume

* Fix copy and paste error

* Enable spherical raycasting again

* Uniform naming

* Add some documentation and remove dead code

data/assets/examples/volume/generated/.gitignore

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+cartesian/
+cartesiansequence/
+spherical/

data/assets/examples/volume/generated/cartesian.asset

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+-- This asset requires OpenSpace to be built with the OPENSPACE_MODULE_VOLUME enabled
+
+-- Before using this example,
+-- the volume data itself needs to be generated,
+-- using the task 'data/tasks/volume/generate_cartesian.task'
+
+local assetHelper = asset.require('util/asset_helper')
+local transforms = asset.require("scene/solarsystem/sun/transforms")
+
+local sunRadius = 695508000
+
+local volume = {
+    Identifier = "GeneratedVolume",
+    Parent = transforms.SolarSystemBarycenter.Identifier,
+    Renderable = {
+        Type = "RenderableTimeVaryingVolume",
+        SourceDirectory = asset.localResource("cartesian"),
+        TransferFunction = asset.localResource("../transferfunction.txt"),
+        StepSize = 0.01,
+        MinValue = 0,
+        MaxValue = 1,
+        GridType = "Cartesian",
+        SecondsBefore = 50*365*24*60*60, -- 50 years before
+        SecondsAfter = 50*365*24*60*60 -- 50 years after
+    },
+    GUI = {
+        Path = "/Examples"
+    },
+    Transform = {
+        Scale = {
+            Type = "StaticScale",
+            Scale = 1000 * sunRadius
+        }
+    }
+}
+
+local objects = { volume }
+assetHelper.registerSceneGraphNodes(asset, objects)

data/assets/examples/volume/generated/cartesiansequence.asset

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+-- This asset requires OpenSpace to be built with the OPENSPACE_MODULE_VOLUME enabled
+
+-- Before using this example,
+-- the volume data itself needs to be generated,
+-- using the task 'data/tasks/volume/generate_cartesian_sequence.task'
+
+local assetHelper = asset.require('util/asset_helper')
+local transforms = asset.require("scene/solarsystem/sun/transforms")
+
+local sunRadius = 695508000
+
+local volume = {
+    Identifier = "GeneratedVolume",
+    Parent = transforms.SolarSystemBarycenter.Identifier,
+    Renderable = {
+        Type = "RenderableTimeVaryingVolume",
+        SourceDirectory = asset.localResource("cartesiansequence"),
+        TransferFunction = asset.localResource("../transferfunction.txt"),
+        StepSize = 0.01,
+        MinValue = 0,
+        MaxValue = 1,
+        GridType = "Cartesian",
+        SecondsBefore = 50*365*24*60*60, -- 50 years before
+        SecondsAfter = 50*365*24*60*60 -- 50 years after
+    },
+    GUI = {
+        Path = "/Examples"
+    },
+    Transform = {
+        Scale = {
+            Type = "StaticScale",
+            Scale = 1000 * sunRadius
+        }
+    }
+}
+
+local objects = { volume }
+assetHelper.registerSceneGraphNodes(asset, objects)

data/assets/examples/volume/generated/spherical.asset

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+-- This asset requires OpenSpace to be built with the OPENSPACE_MODULE_VOLUME enabled
+
+-- Before using this example,
+-- the volume data itself needs to be generated,
+-- using the task 'data/tasks/volume/generate_spherical.task'
+
+local assetHelper = asset.require('util/asset_helper')
+local transforms = asset.require("scene/solarsystem/sun/transforms")
+
+local astronomicalUnit = 149597870700
+
+local volume = {
+    Identifier = "GeneratedVolume",
+    Parent = transforms.SolarSystemBarycenter.Identifier,
+    Renderable = {
+        Type = "RenderableTimeVaryingVolume",
+        SourceDirectory = asset.localResource("spherical"),
+        TransferFunction = asset.localResource("../transferfunction.txt"),
+        StepSize = 0.01,
+        MinValue = 0,
+        MaxValue = 1,
+        GridType = "Spherical",
+        SecondsBefore = 50*365*24*60*60, -- 50 years before
+        SecondsAfter = 50*365*24*60*60 -- 50 years after
+    },
+    GUI = {
+        Path = "/Examples"
+    },
+    Transform = {
+        Scale = {
+            Type = "StaticScale",
+            Scale = astronomicalUnit
+        }
+    }
+}
+
+local objects = { volume }
+assetHelper.registerSceneGraphNodes(asset, objects)

data/assets/examples/volume/transferfunction.txt

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+width 1024
+lower 0.0
+upper 1.0
+mappingkey 0.0 250 250 250  0
+mappingkey 1.0 200 200 200  255