update gadget/gizmo front ends to only build from PartType0

[dnarayanan]

notes in yt slack:

YTOctree(left_edge=None, right_edge=None, n_ref=32, ptypes=None, ds=None, field_parameters=None)

where:

 |  ptypes: list
 |      This is the type of particles to include when building the tree. This
 |      will default to all particles.

[dnarayanan]

based on conversations with @aussing and tied to this issue:

#146

this will hopefully dramatically reduce the computation time for very large datasets

[dnarayanan]

update - this is the magic line:

octree = ds.octree(left_edge = None, right_edge =None, ptypes=['PartType0'])