Add values at bounds into `Timeseries`

The problem

While writing !55 (merged), it became clear that we actually need to know the value of our timeseries at the bounds in order to do differentiation and integration in the general case.

Definition of "done"

• Change Timeseries % values so it is also a container. This container should have values and values_at_bounds attributes for holding the values

Additional context

!55 (comment 1772649390)