Initialize segments with start and end times.

Before the segment duration was provided and zero start time was assumed.
Now initializing and sampling segments have a more consistent API:
- You initialize a segment from start and end time-state pairs.
- You sample a segment with a time[in] and a state[out].
Add doc configuration file.
Add unit tests for segment sampling classes.
First trajectory interface functions with tests.

- Find the segment associated to a specified time instant.
- Sample a trajectory at a specified time instant.