Allow Volume module to work with cell data

The volume module (and some others...) don't seem to work with cell data, only point data. The volume module is hard coded to look for point data when setting the data range. If you just simply change the input to cell data instead, everything seems to work. Of course this isn't a fix, just perhaps some evidence that more flexibility is required?

e.g.

```
--- old_volume.py 2012-08-22 14:31:39.000000000 -0700
+++ volume.py 2012-08-22 14:30:35.000000000 -0700
@@ -426,7 +426,7 @@

# Set the current range.
input = mm.source.outputs[0]
- sc = input.point_data.scalars
+ sc = input.cell_data.scalars
if sc is not None:
rng = sc.range
else:
```