Pr/trails (#170)

* Implement new RenderableTrails as abstract base class
  - Implement RenderableTrailsOrbit and RenderableTrailsTrajectory as concrete instances
Remove old RenderableTrails and RenderableTrailsNew classes
Adapt mod files to the new structure

* Addressed Pull Request comments

data/scene/earth/earth.mod

@@ -55,14 +55,18 @@ return {
         Name = "EarthTrail",
         Parent = "SolarSystemBarycenter",
         Renderable = {
-            Type = "RenderableTrail",
-            Body = "EARTH",
-            Frame = "GALACTIC",
-            Observer = "SUN",
-            RGB = { 0.5, 0.8, 1.0},
-            TropicalOrbitPeriod = 365.242,
-            EarthOrbitRatio = 1,
-            DayLength = 24
+            Type = "RenderableTrailOrbit",
+            Translation = {
+                Type = "SpiceTranslation",
+                Body = "EARTH",
+                Observer = "SUN"
+            },
+            Color = { 0.5, 0.8, 1.0 },
+            -- StartTime = "2016 JUN 01 12:00:00.000",
+            -- EndTime = "2017 JAN 01 12:00:00.000",
+            -- SampleInterval = 3600
+            Period = 365.242,
+            Resolution = 1000
         },
         GuiName = "/Solar/EarthTrail"
     },