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

src/rendering/transferfunction.cpp

@@ -93,7 +93,7 @@ ghoul::opengl::Texture& TransferFunction::getTexture() {
 }
 
 void TransferFunction::update() {
-  /*  if (_needsUpdate) {
+    if (_needsUpdate) {
         if (hasExtension(_filepath, "txt")) {
             setTextureFromTxt();
         } else {
@@ -104,7 +104,7 @@ void TransferFunction::update() {
         if (_tfChangedCallback) {
             _tfChangedCallback(*this);
         }
-    }*/
+    }
 }
 
 void TransferFunction::setCallback(TfChangedCallback callback) {