* Graph3D: move Filter into DataGroup

The `Filter` instance within `Graph3d` is intimately connected to the graph data,
contained in a `DataGroup` instance. As such, it needs to be placed within `DataGroup`.

A consequence of this is that, in the final case of multiple graphs, each graph can be
animation separately. I regard this as an advantage, even though it will means more
initialization for the graphs (you have to initialize each separately for an animation.

An effort has been made to decouple `Graph3d` and `Filter` as much as possible. There
are still some relationships present, but it's more bother than it's worth to dissolve these.

In addition to moving the `Filter` instance, the following has been done:

- Added variable `style` to `DataGroup`
- Moved certain data-specific methods from `Graph3d` to `DataGroup`
- cleaned up some code and commenting

These changes have been tested with the following examples:

- `graph3d/10_styling`
- `graph3d/03_filter_data`
- `graph3d/04_animation`

* Add changes to filter

* First working version of eslint enabled with gulp

* Tryout of eslint with graph3d files

* Cleanup of gulp and eslint files.

* Completed test by linting graph3d

* Disable travis linting for now

* Remove global cmd param from .travis.yml

* First working version of new class DataGroup.

* Adjustments for review - done all points except last

* DRY distinct values, clean up sort code

* Added missing @param's to comments in DataGroup