Move timeseries expressions to ODE

[robfalck]

Proposed feature.

TImeseries expressions are useful but a more streamlined way to implement them is to include the ExecComp in a group with the ODE.

Once the Radau and GaussLobatto refactors are complete this should be possible and allow for the removal of a good bit of the complexity in timeseries.

Example

There will be no change in the user-facing API.