Instructions in README not clear

I followed the instructions in the README, i.e., I did a 
```bash
python3 setup.py install
```
first, which produced sane output.

But when I run `from interp3d import interp_3d`,
I still get 
```
  File "/home/......../interp3d/interp_3d.py", line 1, in <module>
    from .interp import _interp3D
ModuleNotFoundError: No module named 'interp3d.interp'
```