Handle final time minimization problems

[tmigot]

There are also constraints on this unknown final time.

[tmigot]

The problem is that everytime I evaluate the function at a special point, we get

Evaluation of a CellField at a given Point is not implemented yet.

This is a feature that we want to have at some point in Gridap.
If you are ready to help with this implementation, please contact the
Gridap administrators.