Requirement of tibbles for new_tdcmm class

[LKobilke]

Some of our new functions require the tibble format for them to work properly. We should catch that errror with an if statement and transform the input data to a tibble before running the function. Here's an example:

[MarHai]

That would actually be a nice fix together with #49 when streamlining code (i.e., moving such requirement tests into a separate function/file).