Implement and test explicit coupling

[BenjaminRodenberg]

To support explicit coupling, some things still have to be implemented. Generally, there are two flavours that we would like to support:

• FEniCS participant only writes data. See also Offers more control to the user via new interface #59 (comment)
• FEniCS participant only reads data.

[BenjaminRodenberg]

In theory: Should work out of the box.
In practice: Needs to be tested!

[BenjaminRodenberg]

I did a quick test using the partitioned heat example with the following modified configuration file:

...
      <coupling-scheme:serial-explicit>
         <participants first="HeatDirichlet" second="HeatNeumann"/>
         <max-time value="1"/>
         <time-window-size value=".1" valid-digits="8"/>
         <exchange data="Flux"        mesh="NeumannNodes" from="HeatDirichlet" to="HeatNeumann" />
         <exchange data="Temperature" mesh="NeumannNodes" from="HeatNeumann"   to="HeatDirichlet" initialize="true"/>
      </coupling-scheme:serial-explicit>
...

I did not have to modify the adaper or heat.py (only comment out error checks) to run this case. The result looks like expected:

Due to explicit coupling the contour lines are not continuous and we do not get the exact solution anymore.

I will keep this issue open, since the two point mentioned above are still untested:

• FEniCS participant only writes data. See also #59 (comment)
• FEniCS participant only reads data.

[BenjaminRodenberg]

I quickly put together a one-way coupling variant of the partitioned heat example, where the Neumann solver only writes and the Dirichlet solver only receives (see one-way.zip).

Generally, it works. However, I observed an issue:

precice_dt = precice.initialize(coupling_boundary, mesh, V_g, write_function=u_D_function)

initialize always requires a function space. Even, if the participant does not read any data. However, for one-way coupling the function space should be optional, since it is not needed by the participant that writes.

I propose the following signature for initialize:

precice_dt = precice.initialize(coupling_boundary, mesh, read_function_space=V_g, write_function=u_D_function)

Related to #71.

[IshaanDesai]

I propose the following signature for initialize:

precice_dt = precice.initialize(coupling_boundary, mesh, read_function_space=V_g, write_function=u_D_function)

Related to #71.

This is being handled in #71 as having separate function spaces for read and write functions was already discussed there.

[BenjaminRodenberg]

Let's do a final verification of this functionality before merging #71 and then close this issue. No need for a unit test.

[BenjaminRodenberg]

Did another test of explicit and one-way coupling. one-way.zip

Everything works. I just found a small bug that is fixed in #107