dust masses for gizmo with OTF dust

[dnarayanan]

right now the dust masses for gizmo/gadget datasets with on-the-fly dust won't work for manual mode because it will look for PartType0.

a possible way to fix this would be:

1. in front_ends/gadget2pd.py in _dustsmoothedmasss, have a clause if otf_extinction is set to change:

        if yt.__version__ == '4.0.dev0':
            dsm = ds.arr(data.ds.parameters['octree'][('PartType0','Dust_Masses')],'code_mass')
            #return (data.ds.parameters['octree'][('PartType0','Dust_Masses')])
            return dsm
	else:
            return (data.ds.arr(data[("deposit", "PartType0_sum_Dust_Masses")].value, 'code_mass'))```
            
            to take `PartType3` instead.   I think this should deposit the dust masses into the octree (we may need to include a ```ds._sph_ptypes=('PartType0','PartType3')``` line in there as well)

[dnarayanan]

side note this issue existed for arepo too, though the arepo part was fixed in commit 996d8689f3016f5db231d16146f01dcc76e8c8fe in the variable_extinction branch