fftw3 example not able to fetch files from conda repo

Hi,

I tried the fftw example to containerize a conda environment. However, it gives an error that it cannot find `fftw3` in the channels. I have used the same def.yml file given in the example.

I also tried to replace `fftw3` by `fftw`, it downloads the file but gives error in running the post install script:

```
cp: cannot stat 'fftw.cpp': No such file or directory
```