Partitioned heat conduction: initialize data for Nutils participants

[uekerman]

The Nutils participants don't initialize data correctly:

tutorials/partitioned-heat-conduction/nutils/heat.py

Lines 103 to 105 in 017b97c

	if interface.is_action_required(precice.action_write_initial_data()):
	precice_write(coupling_sample.eval(0.))
	interface.mark_action_fulfilled(precice.action_write_initial_data())

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

tutorials/partitioned-heat-conduction/nutils/heat.py

Line 36 in 017b97c

ns.uexact = '1 + x_0 x_0 + alpha x_1 x_1 + beta ?t' # analytical solution

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

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