[WIP] Refactor show, snapshot and exposed add_dataset function to plugin

[kitchoi]

• [x] Expose the add_dataset function as a shortcut to setting up a CUDS Source for mlab.pipeline.add_dataset.
• [x] Add add_default_modules.

Refs: #134

Usage:

from simphony.visualisation import mayavi_tools
source = mayavi_tools.add_dataset(cuds, point_scalars_name="TEMPERATURE")  # scene is empty
mayavi_tools.add_default_modules(source)   # add default visualisation

show is doing exactly the above + mlab.show() Usage for show (similarly for snapshot):

from simphony.visualisation import mayavi_tools
mayavi_tools.show(cuds, point_scalars_name="TEMPERATURE", point_vectors_name="VELOCITY")

# if "TEMPERATURE" and "VELOCITY" are the only point scalars and vectors, 
# previous behaviour works fine
mayavi_tool.show(cuds)

• [ ] Need to update documentation

@itziakos, @dpinte what do you think?

After this PR is accepted and merged, I will create a new issue for making the GUI use these functions instead.

[kitchoi]

Relevant to #132 too

[kitchoi]

@dpinte, @itziakos, this is ready for review. Please let me know if this is good to merge.

[kitchoi]

add_dataset may not be ideal. Let me try a different implementation for refactoring.

[dpinte]

@kitchoi LGTM

[kitchoi]

Superseded by #162, closing.