Filtering 3D viewer: phase 1

We need to add a "Color by:" dropbox on the histogram that controls what is displayed in the histogram and the 3D viewer. By default, the dropbox is going to contain "CT" (i.e., the original data).

Values in the dropbox are given by a list of `Filter` objects that have a name (to be displayed in the dropbox) and a function, `filter`, that takes the 3D volume data and return a new 3D array with the same shape and potentially new data.

The output of the `Filter` object is used to compute a new histogram, and to update the content of the 3D view.

Things we need to care about:
- [ ] ~~The dropbox responds to changes to the list of filters~~
- [ ] ~~`Filters` have an `updated` `Event` trait; the histogram listens to that trait and when it is fired it calls the filter again to recompute its output~~
- [x] Update/add tests for `VolumeData`
- [ ] Refactor the masking functionality into a `VolumeFilter` subclass. See: https://github.com/enthought/ensemble/issues/46#issuecomment-147073792