Additional error handling and testing

[SBlokhuizen]

• Added additional checks for the annotations objects of the tendencies. Currently it is only checked if the annotations are empty if we do not expect an error, and filled if we do expect an error. This can be done more rigorously in the future if we settle on specific error messages.
• tests for waveform.py and annotations.py are introduced.
• There were issues with the interpolated values of derivatives of piecewise tendencies, which are now fixed and tested.
• Gaps and overlapping tendencies are now explicitly handled and tested. Gaps between tendencies are interpolated, whilst derivatives at gaps are set to 0.
• The read_yaml_from_file method is updated to also use annotations.
• Issue is solved where the first tendency of a repeated tendency would give an error, as it's previous tendency is set to the last tendency in the repeated tendency, causing the start to be later than previous' end.

[maarten-ic]

Hmm, by doing this only during set_previous_tendency, we lose the declarative nature of updating the user_ parameters and let everything update itself (without re-instantiating all objects).

Though this is the case for all error handling (because any issue is just added to the existing annotations)...

Let's discuss? It works now, but the mix of some declarative logic (e.g. _calc_times) mixed with imperative logic (e.g. this method) could be confusing