Check dependies in functions

[mumichae]

Check if optional dependencies are available in their respective functions and control behaviour when import fails.

Checks for:

• [x] all integration methods
• [x] kBET
• [x] LISI

[mumichae]

The test is tailored to the Github actions workflow. In order for kBET to work properly, we'd need a proper R installation in the CI, which would be something for a separate workflow and PR.

[LuckyMD]

Thanks @mumichae !