Liquid Simulation Display Options (GSoC 2020)

All the changes made in the branch `soc-2020-fluid-tools` are included in this patch.

**Major changes:**

=== Viewport Display ===

- //Raw voxel display// or //closest (nearest-neighbor)// interpolation for displaying the underlying voxel data of the simulation grids more clearly.
- An option to display //gridlines// when the slicing method is //single//.

==== Grid Display ====

- Visualization for flags, pressure and level-set representation grids with a fixed color coding based on Manta GUI.

==== Vector Display ====

- //**M**arker **A**nd **C**ell// grid visualization options for vector grids like velocity or external forces.
- Made vector display options available for external forces.

==== Coloring options for //gridlines// ====

- Range highlighting and cell filtering options for displaying the simulation grid data more precisely.
- Color gridlines with flags.

- Also, made slicing and interpolation options available for Volume Object.

Reviewed By: JacquesLucke, sebbas

Differential Revision: https://developer.blender.org/D8705

release/scripts/startup/bl_ui/properties_data_volume.py

@@ -142,6 +142,9 @@ class DATA_PT_volume_viewport_display(DataButtonsPanel, Panel):
 
         volume = context.volume
         display = volume.display
+        axis_slice_method = display.axis_slice_method
+
+        do_full_slicing = (axis_slice_method == 'FULL')
 
         col = layout.column(align=True)
         col.prop(display, "wireframe_type")
@@ -149,7 +152,14 @@ class DATA_PT_volume_viewport_display(DataButtonsPanel, Panel):
         sub.active = display.wireframe_type in {'BOXES', 'POINTS'}
         sub.prop(display, "wireframe_detail", text="Detail")
 
-        layout.prop(display, "density")
+        col = layout.column(align=True)
+        col.prop(display, "density")
+        col.prop(display, "interpolation_method")
+        col.prop(display, "axis_slice_method")
+        
+        if not do_full_slicing:
+            col.prop(display, "slice_axis")
+            col.prop(display, "slice_depth")
 
 
 class DATA_PT_custom_props_volume(DataButtonsPanel, PropertyPanel, Panel):