fix to setup.py to look for numpy headers

[bw4sz]

Thanks for all your great work @qianguih and @jeasinema. I found it useful to generalize the setup.py to look for the numpy headers when building the extension. Without this, user might need to manually set where the numpy numpy/arrayobject.h file is, which can be tricky in the virtual env. This should make it more robust. Tested on the University of Florida computing cluster.

(voxelnet) [b.weinstein@dev1 voxelnet]$ python
Python 3.6.5 |Anaconda, Inc.| (default, Mar 29 2018, 18:21:58) 
[GCC 7.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

(voxelnet) [b.weinstein@dev1 voxelnet]$ python3 setup.py build_ext --inplace
running build_ext
building 'utils.box_overlaps' extension
...

Running

(voxelnet) [b.weinstein@dev1 voxelnet]$ cat /proc/version
Linux version 3.10.0-693.17.1.el7.x86_64 ([email protected]) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-16) (GCC) ) #1 SMP Sun Jan 14 10:36:03 EST 2018

inside a conda env.