Immediately create object on initialisation in API

[BradyAJohnston]

Related to #762

Currently the parsing of the data and the creation of an object inside of Blender are separate processes. I think these should instead be one and the same. If a Molecule or Trajectory exist, then they should have a corresponding object that represents all of that data in the scene.

Rather than the current approach of creating an object with a style parameter, we should instead add styles to an already created object.

# old
mol.create_object(style='ribbon')

# new
mol.add_style(type='ribbon', selection='a_named_attribute_selection')
mol.add_style(type='ball_and_stick', selection='is_nucleic')

Requires creation of internal node logic for editing node trees, but this shouldn't be too difficult.