Custom Newton solver for mixed element spaces

[dokken]

There are many questions/answers regarding Mixed elements on the forum (both for legacy dolfin and dolfinx).

@sbhasan

In my case the mesh will evolve from one time step to another. Since the feature is not available in 0.5.x, I will have to interpolate manually the dofs from old mesh on to new one.

Then you must collapse the function into its sub space (and use the map stemning from FunctionSpace.collapse() to map from the sub space to the parent space.
See for instance: Assigning a scalar expression to mixed function space - #4 by dokken