Focus CI on Custom-KinFragLib

Description

Running the full CI for Sonja's project seems to waste resources.

Remove all tests on the original notebooks until we merge back to volkamerlab/kinfraglib.

Notable points that this PR has either accomplished or will accomplish.

[x] Remove CI for original notebooks by specifying new notebooks only in pytest command
[x] Remove zenodo download (remove job)
[x] Open an issue to remind us to add back all tests before merging to volkamerlab/kinfraglib: https://github.com/NonaR/KinFragLib/issues/6
[x] Add this issue as a TODO to PR #2
[x] Add custom-base branch to workflow to trigger PR that merges back to custom-base
[x] Change cd .. plus pip install -e KinFragLib to python setup.py install to avoid changing directories (i.e. staying in KinFragLib/)

None.