Instructions in README not clear

[baloe]

I followed the instructions in the README, i.e., I did a

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'