Installing avisynth on Ubuntu 16.04

[Selur]

I am assuming that vapoursynth does not support this

Vapoursynth does support this and even Hybrid without Avisynth or Vapoursynth can do that using "Filtering->Speed Change->Interpolate frame rate" which uses the ffmpeg frameratefilter-filter from ffmpeg.
That said, the Avisynth and Vapoursynth frame rate interpolation filters offer better quality and more options.

That side, you should not have to install the avisynthExtension manually.
Adding the repository, updating the packages using:

sudo add-apt-repository ppa:djcj/hybrid
sudo apt-get update

and using:

sudo apt-get install hybrid hybrid-tools hybrid-avisynth-extension hybrid-vapoursynth-extension

should install all needed stuff for Hybrid with Avisynth and Vapoursynth support.

"Vapour->Other->Frame Interpolation" offers two different methods for interpolation, 'Interframe/SVP' and 'MVToolsFPS', I hope djcj included the required dependencies for it (haven't checked).

Cu Selur