Partitioned heat conduction: initialize data for Nutils participants

[uekerman]

The Nutils participants don't initialize data correctly:

https://github.com/precice/tutorials/blob/017b97cff4a9e03d7062a8ef1a076edf78b0a6b1/partitioned-heat-conduction/nutils/heat.py#L103-L105

We should instead sample uexact (or its flux) defined further above:

https://github.com/precice/tutorials/blob/017b97cff4a9e03d7062a8ef1a076edf78b0a6b1/partitioned-heat-conduction/nutils/heat.py#L36

Maybe this also the reason for this FEniCS - Nutils combination issue.

Problem applies to partitioned-heat-conduction and partitioned-heat-conduction-direct.