pspy requiring ducc but not installing it as a dependency

[sgiardie]

pspy requires ducc here, but it does not require its installation

[xgarrido]

We didn't want to add ducc as a pspy dependency since the fft computation is a temporary fix until pixell properly integrates ducc. This is now the case in 0.19 version release of pixell (fft support as well as alm computation). We didn't have time to check pspy against pixell 0.19 but it should be pretty straightforward. So, if you want to use this https://github.com/simonsobs/pspy/blob/master/pspy/fft.py#L35 then you have to install ducc by yourself pip install ducc0