Add additional warning messages to engines with regards to assumptions and applicability

[Martin-Jung]

Many engine in the package can be particular data hungry or make certain assumptions about the input data provided to the package. Further messages via myLog in a highlighted colour should be added directly to the engines to make users aware of any violations of assumptions.

Update:
Additionally or alternatively, a better idea would be to add a simple wrapper that tests common model assumptions implemented in a specific function check(fit).

• Implement such a function (check) in a seperate call.
• Implement engine and parameter specific posthoc checks.
• Add a test

[Martin-Jung]

Closing for now with 4179be6. Further checks to be added on demand...