Add interpolation from conda-forge

[mmcky]

interpolation package has been added to conda forge so we can use that channel to install

!conda install -y quantecon
!pip install interpolation

should become

!conda install -y quantecon
!conda install -c conda-forge -y interpolation

[mmcky]

@jstac we would need to specify the conda-forge channel for the interpolation package so I don't really see a lot of benefit in making the change. What do you think?