python 3 fix for indoor3d_util.py

[PeterPyPan]

In python 3 it is not allowed to use the + operator on a range object and a list.
This causes an error in indoor3d_util.py (sample_data(), line 129) when running the gen_h5.py script.

Minimal fix for python 3 compatibility:

>>> return np.concatenate([data, dup_data], 0), range(N)+list(sample)
TypeError: unsupported operand type(s) for +: 'range' and 'list'
>>> return np.concatenate([data, dup_data], 0), list(range(N))+list(sample)
this works

[WXinlong]

Hi, we follow the SGPN for the data processing part and use the same indoor3d_util.py file.
Thank you for pointing out this and it could be helpful for others using python3.