Collection of unit test suggestions

[jschepers]

Here we can collect suggestions for possible unit tests that are currently not implemented:

• Test the case that single components are given in the sequence component dict (instead of a vector) -> make sure that it either works by internally converting to a vector (if not implemented yet) or give an informative error message.
• Test for the case that the keys in the sequence component dict do not match the events in the sequence design or vice versa
• Test whether error is thrown for multi-subject sequence designs
• Test cases for MultichannelComponent and MixedModelComponent with a component function