Automatic refinement of elements

[dbrakenhoff]

As described in #103 this PR implements automatic refinement of elements in TimML.

See the example notebook for a detailed description of the new functionality.

One thing I'm not entirely sure about is the refinement of single line-sinks. This cannot be done without creating new elements, which means the reference to the element original added by the user cannot be used for computations, which might be a bit confusing. I've implemented refinement for these elements now, but we could also choose not to support refinement for single line-sink elements, and point users to their LineSinkString counterparts.

Anyway, curious to get your feedback!