Use papermill library to execute notebooks and check necessary outputs ?

[anutosh491]

We were trying to introduce a testing framework to test out the example notebooks in xeus-cpp and confirm the cell outputs.

• An issue for the same was raised here https://github.com/compiler-research/xeus-cpp/issues/122
• The papermill library was used to fix the same as shown in the PR here https://github.com/compiler-research/xeus-cpp/pull/142

There might be a possibility to move this to xeus and extrapolate this idea to all xeus-kernels.