Restrictions on using adapter for 3D cases

[IshaanDesai]

Functionality to handle 3D cases was introduced in the adapter with https://github.com/precice/fenics-adapter/pull/133, https://github.com/precice/fenics-adapter/pull/146 and https://github.com/precice/fenics-adapter/pull/147

However there are still several restrictions on the use of this functionality. The restrictions are:

1. When quantities on the coupling boundaries are a set of PointSource objects, the adapter can used for 3D cases but it can only be run in serial. Parallel runs using MPI are forbidden. More information regarding this can be found in this Discourse post. It is expected that using FEniCS-X will resolve this problem.
2. When quantities on the coupling boundaries are of the type of an Expression object, the adapter cannot be used in 3D. Only 2D cases are possible in this case. The main reason for this is the way in which we manually construct the interpolant. A more general strategy would help lift this restriction. Check out the issue: https://github.com/precice/fenics-adapter/issues/119 for one such idea.